More than 200 demonstrators gathered outside the US Embassy in London on Tuesday to protest against Washington's proposed military intervention in Syria.
The protesters held placards that read,” Hands off Syria” and “David Cameron beat back, no USA attack".
The demonstration began at 5.30pm (GMT), coinciding with the start of the John Kerry's testimony, the US Secretary of State appearing at the Senate Foreign Relations Committee in Washington to argue the Obama administration's case for using military force against President Bashar Assad.
Labour MP Jeremy Corbyn, who attended the event outside the Grosvenor Square entrance, called on Congress to vote against a military strike.
"I want to give a message to the US political system and people that there's been a big change in Britain and we've finally voted not to follow the US into war on this occasion in Syria. The president has asked Congress to give him the authority or otherwise. I hope they don't give him that authority."
Corbyn continued: "Maybe he's got doubts himself about the sense of this and he's concerned about where it would lead to?"
The protest was organised by campaign group Stop the War Coalition. Chairman Chris Nineham said: “We need to keep the pressure up because it's clear that anti-war mobilisation does make a difference.
Earlier on Tuesday, the United Nations warned Barack Obama that the US should not carry out "punitive" strikes against Syria for its alleged use of chemical weapons.
Ban Ki Moon said an attack to punish Syria would "unleash more turmoil", adding that the UN Charter made illegal any military action unless in self defense or approved by the Security Council.