The officer resigned over comments posted online about the protests
A Metropolitan Police (Met) officer has resigned over inappropriate comments on a website about the G20 protest, acting Deputy Commissioner Tim Godwin said.
Pc John Hayter's resignation was revealed at a public meeting over the policing of G20 protests in London.
Pc Hayter, from the Royal protection unit, allegedly wrote: "I see my lot have murdered someone again."
A Met spokesman said employees would be disciplined if they contribute anything "deemed inappropriate".
At the Metropolitan Police Authority (MPA) meeting in City Hall protesters heckled and jeered Mayor Boris Johnson who said that officers in general had done a "fantastic job".
The Met said Pc Hayter, 49, stepped down after the force began an internal inquiry over the alleged comments posted on Facebook.
A Met spokesman said: "Before his resignation the officer in question was under investigation by the Directorate of Professional Standards for allegedly posting inappropriate comments on a social networking website.
"If any employee is identified as contributing material that is racist, homophobic, sexist, defamatory, offensive, illegal or deemed inappropriate they may be subject to disciplinary procedures."
Another officer, who allegedly wrote on a social networking website that he was keen to "bash some long haired hippies" at the protests, has also been disciplined.
The IPCC is independently investigating Ian Tomlinson's death
Following the G20 protests on 1 and 2 April two officers were suspended, one of whom was interviewed on suspicion of manslaughter by the Independent Police Complaints Commission (IPCC) over the death of newspaper seller Ian Tomlinson, 47, who died apparently after being pushed.
The IPCC is also looking into more than 80 allegations of police using excessive force.
MPA members and the force's board of governors faced a group of protesters holding a large banner reading "Remember Ian Tomlinson" as they arrived at City Hall.
Mr Goodwin was questioned about the containment tactic, known as kettling, police cordons, the use of video footage as evidence, the climate camp and the role of the Territorial Support Group (TSG).
The MPA said the force must make improvements in the way protests are handled in the future.
Mr Goodwin said: "It is the responsibility of all police officers to use reasonable and proportionate force and all police officers have to account for that use.
"This was the biggest security operation mounted by the Met for many years. But the vast majority of officers acted professionally and at times with great courage to facilitate lawful protest and to allow G20 to go ahead."
Protesters heckled the speakers including the mayor when he defended officers on duty during the G20 summit.
Mr Johnson said: "Everybody in London and in the country has of course been horrified by some of the images of what happened, in particular to Ian Tomlinson.
"But I hope members will agree that it is clear that the overwhelming majority of police officers in London today and indeed at the G20 are doing a fantastic job."