LONDON — 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 (IS) jihadists.

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

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 central London carrying placards reading "Don't bomb Syria," "Drop Cameron, not bombs" and "Don't add fuel to the fire."

"David Cameron's incoherent proposals for action in Syria will do nothing to weaken Isis but will instead inflame the civil war, deepen the misery of the Syrian people and increase the terrorist risk," said the Stop the War Coalition protest movement.

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.

Some 5,000 people also protested in Madrid against military action in Syria, 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.

Convincing Case?

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

On Thursday Defence 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 IS 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.

Britain's potential participation in Syria airstrikes has proven deeply divisive, with Cameron having lost a parliamentary ballot on military action against Syrian leader Bashar al-Assad in 2013, leading to a humiliating climbdown.

He now insists he will not hold a vote until he is sure he has enough support.

The main opposition Labour party is also deeply divided, with the vote threatening to tear the party apart and undermine leader Jeremy Corbyn.

Long-time anti-war campaigner Corbyn is opposed to airstrikes, but several members of the party have signalled 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.