Former Labor cabinet minister Tony Benn and veteran campaigner Brian Haw joined the launch of the 'Vigil for Gaza' in Parliament Square on Thursday, where Haw has been camped in protest against the Iraq war since 2001.
"Nowadays the real politics of the world is outside parliament and not inside," said Benn, who is now president of the Stop the War Coalition, Britain's umbrella peace organization, the Islamic republic news agency reported.
The 83-year old former politician, who was an MP for almost 50 years, condemned the "massive humanitarian crisis" created by Israel in Gaza.
Emma Sangster, one of the vigil's organizers, said the rally will "continue 24 hours a day, every day, until the situation substantially changes, until Israel pulls out of Gaza and also until the UK government makes an effective response to the situation."
"We feel that only Israel is allowed to get away with such a low-level response from the international community," Sangster said.
The British parliament has remained in recess for the Christmas period despite calls for MPs to return to debate Israel's latest slaughter of nearly 800 Palestinians in Gaza.
The Liberal Democrats have called for an emergency debate when parliament reconvenes on Monday, but this is not expected to be held until next Thursday at the earliest.
"Many people in Britain are viewing events in Gaza with increasing alarm and despair," the Lib Dem's shadow leader of the House of Commons, David Heath, warned.
The vigil comes as police are braced for further protests being staged around Britain, with major demonstrations again taking place in London and Edinburgh on Saturday.
Daily rallies have been held outside the Israeli Embassy in London since Israel launched its latest wave of killings in Gaza nearly two weeks ago, local protests have also been held in dozens of towns and cities across the UK.