It has become a defining image of the London G-20 summit -- a video of the last moments of Ian Tomlinson's life.
Mr. Tomlinson, a 47-year-old newspaper vendor, is seen walking down a street in the center of the city on the evening of April 1, trying to navigate police lines and crowds of protesters on his way home. His hands are in his pockets.
A dozen police officers in yellow jackets approach him from behind. One officer, wearing a black ski mask, slaps Mr. Tomlinson's legs with a baton. The officer then shoves him.
Mr. Tomlinson falls, slamming against the ground.
"What did he do?" a young protester shouts at police.
Mr. Tomlinson then stands and walks away. Up the road, off camera, he collapses.
The video, filmed by an American and later obtained by the Guardian newspaper, sparked a public uproar in the United Kingdom about police handling of protests during the most recent summit.
As they prepare to host the next G-20, Pittsburgh police officials are looking closely at what happened in London and at similar gatherings, such as the meeting of the World Trade Organization in Seattle in 1999.
"We have gotten a lot of great information and insight that we have incorporated into our planning process," said Diane Richard, a spokeswoman for the city police bureau.
At first, police officials in London claimed that Mr. Tomlinson had suffered a heart attack and protesters had pelted officers with bottles as they tried to help him. But an autopsy showed that Mr. Tomlinson died from internal bleeding, and, in the video, no officers were seen assisting him after he hit the ground.
Mr. Tomlinson was the only fatality during the summit, but scores of other videos have emerged, showing violent clashes between police and protesters on the streets of London on April 1 and 2.
Prosecutors are considering manslaughter charges against the officer who struck Mr. Tomlinson, and another officer has been suspended for hitting two women.
Police also have faced criticism for the use of "kettling," or surrounding and containing groups of protesters in one area for long periods of time. Britain's Independent Police Complaints Commission has received 280 complaints related to the G-20.
The controversies have led to several investigations and calls for reform in the United Kingdom in how police handle protests.
"These incidents and the tactics that led to them caused considerable adverse comment and have the potential to seriously damage the public's faith in the police," said a House of Commons report released in June.
The report, which followed weeks of public hearings, praised police for organizing a "remarkably successful" security operation under trying circumstances. But it also argued that officers need to emphasize the protection of democratic rights and improve communication with protest groups.
"Above all, the police must constantly remember that those who protest on Britain's streets are not criminals but citizens motivated by moral principles," it said. "The police's doctrine must remain focused on allowing the protests to happen peacefully."
Pittsburgh public safety officials and the Secret Service -- who are overseeing security for the next summit Sept. 24 and 25 at the David L. Lawrence Convention Center -- face the same delicate balance of ensuring constitutional freedoms of speech and assembly versus protecting a high-profile gathering of world leaders.
The differences between the settings of the London and Pittsburgh summits are vast.
London, with more than 7 million inhabitants, is a national capital and a center of global finance. Its Metropolitan Police Service, also known as the "Met," has 31,000 officers, while the Pittsburgh Police Bureau has fewer than 900 officers.
And Londoners are accustomed to seeing angry citizens air grievances on the streets. According to police statistics, more than 5,300 protests were held in the greater London area between April 2008 and March of this year.
But the two summits have a critical similarity: Organizers had only a few months to assemble massive security operations.
"I would say that this week of the G-20 was probably the most complex policing event the Metropolitan Police and our partners have undertaken, certainly in my length of service," police Cmdr. Bob Broadhurst told a House of Commons committee hearing in May.
The Met commissioner, Sir Paul Stephenson, called it "a remarkable operation planned over an incredibly short period of time."
Even with London's prominence as a spot for demonstrations, very few of its officers had been through anything of the magnitude of the G-20, which attracted an estimated 35,000 protesters. Many frontline officers were beat cops from London's boroughs who had received just two days of crowd-control training a year.
On April 1, the first day of the London summit, 10 separate protests were held at seven sites, including a "stop-the-war" march that took place without incident, according to a report from Her Majesty's Chief Inspector of Constabulary, or HMIC.
About 4,000 to 5,000 demonstrators gathered at the Bank of England. The size of the crowd surprised police, who called for reinforcements from the ExCel Centre, the main site for G-20 meetings.
By noon, officers were facing flying bottles. One officer collapsed after a pole hit his head. Some protesters attacked the Royal Bank of Scotland.
Police commanders decided to impose an "absolute cordon," meaning thousands of people -- including journalists and passers-by who were not involved with the protests -- would be held in one spot.
The encounter with Mr. Tomlinson occurred in this area around 7 p.m.
At the same time, a peaceful gathering of environmental activists known as "Climate Camp" at nearby Bishopsgate became chaotic when police decided to impose another containment area, confining 4,000 protesters.
According to one report, police commanders were worried about the violence from the bank area leaking into the Climate Camp group. Also, that gathering had not been approved by police and was blocking a roadway.
Later in the evening, police began to move the environmental protesters again, with some officers using shields, batons and gloved hands to strike people directly in front of them.
"There was no warning given. There was no request to move. There was no indication of what was going to happen," Chris Abbott, who was in the crowd, told legislators. "The police advanced on us. They pressure pointed my girlfriend on the neck ... and dragged her backwards off me."
One officer punched Mr. Abbott in the face. Others started hitting him with shields, he said.
"The police 'kettled' the Climate Camp for no obvious reason ... they also physically compressed the protest into a smaller and smaller space, using violence to do so," David Howarth, a member of Parliament and protest observer, wrote in an e-mail message last week.
"There was no reason either of public order or public safety for this maneuver, and it was only the remarkable composure and presence of mind of the protesters that prevented the situation from deteriorating into a very dangerous riot."
Another highly publicized confrontation caught on video occurred April 2 at a vigil for Mr. Tomlinson, when a police sergeant pushed and smacked protester Nicola Fisher.
She pushed the sergeant and shouted, "What are you doing hitting a [expletive] woman?"
The sergeant smacked her again and hit her legs with his baton, shouting "Get back!"
Cmdr. Broadhurst told lawmakers that most officers used restraint during the London protests, but the lack of experience was a reason "one or two of them, as you have seen on television, may have used inappropriate force at times."
The House of Commons report said inexperienced officers should "never again" be placed on the frontlines during events such as the G-20.
Frances Wright, a legal observer for Climate Camp, said rank-and-file officers need to change how they think about large demonstrations.
"Very often officers are brought in to police an area they don't know and they assume there's going to be problems," Ms. Wright said in a phone interview last week. "They don't really come with the attitude that they are supposed to be facilitating the protest."
That attitude already has started to change, she said. Last week, London was the site of another Climate Camp protest, and the police presence was much smaller. There were no major incidents, Ms. Wright said.
Both sides agree that communication between police and protesters is crucial, before and during the events.
Mr. Howarth warned that "misguided police tactics" could transform a peaceful protest -- even ones that may not have proper permits or permission from authorities -- into a violent event.
"The starting point for policing protest should be that there is a democratic right to protest peacefully and that the main objective of the police, in a democracy, should be to facilitate peaceful protest," he said, "not to prevent or disrupt it."