Airstrikes in Syria
Airstrikes in SyriaReuters

Some 5,000 people protested in London Saturday against potential British participation in Syria airstrikes, as political momentum mounted to broaden the fight against Islamic State (ISIS) jihadists.

Prime Minister David Cameron on Thursday laid out the case for British jets, already bombing ISIS targets in Iraq, to join France, the United States and others in targeting ISIS strongholds in neighboring Syria.

A parliamentary vote on bombing Syria is expected as early as next week, and many formerly reluctant politicians are thought to have changed their minds after the Paris attacks.

In 2013 Cameron suffered a defeat in parliament when MPs voted against British military action against the Assad regime in Syria to deter the use of chemical weapons.

Yet Britain remains deeply scarred by its former interventions in Afghanistan and Iraq, the latter drawing hundreds of thousands of protesters onto the streets of London in 2003.

In an echo of that protest, thousands gathered in the British capital carrying placards reading "Don't bomb Syria", "Drop Cameron, not bombs", and "Don't add fuel to the fire".

"There was a terrible tragedy in Paris but it's the job of a responsible government to respond to that in a responsible way, and not just simply say that they're going to bomb," Stop the War Coalition's Lindsey German told AFP.

"We're saying ... don't do this, don't make the same mistake you made with the Iraq war."

Speaking in Malta on Saturday, Cameron reiterated his view that ISIS is a threat to Britain and conducting airstrikes in Syria would be the "right thing for Britain to do".

Some 5,000 people also protested in Madrid against military action, with many wary of Spain becoming a target for militants again after Al-Qaeda-inspired bombers blew up commuter trains in the Spanish capital in 2004, killing 191 people.

Many Spaniards believe the attack was in retaliation for their country's involvement in the Iraq war.

With December 20 polls fast approaching, Prime Minister Mariano Rajoy's conservative government has held off on any decision.

French leaders, still reeling from the coordinated ISIS gun and bomb assault that killed 130 people on November 13, have in recent days called on allies to join France in stepping up military action against the jihadist group.

On Thursday, French Defense Minister Jean-Yves Le Drian called on Britain to help "win this war", and in a rare intervention in a British parliamentary ballot, President Francois Hollande on Friday urged lawmakers to "meet the request of Prime Minister Cameron".

A day later, Foreign Minister Laurent Fabius said the campaign against ISIS should move beyond airstrikes to ground troops, through alliances with Arab forces.

"It will be necessary... France has no intention of intervening on the ground. Foreign troops would be seen as an occupying force. Therefore they must be Syrian, Arab, Kurdish troops," he told Spain's El Pais newspaper, the quotes translated from Spanish.

In Britian, the main opposition Labour party is torn, with the vote threatening to fracture the party and undermine leader Jeremy Corbyn.

Corbyn is opposed to airstrikes, but several members of the party have signaled they will rebel amid talk that some could resign over the issue.

In a letter to Labour lawmakers on Thursday, Corbyn said the prime minister had failed to make a "convincing case" for joining the conflict.

AFP contributed to this report.