LONDON - British lawmakers voted by a wide margin Wednesday to join the international campaign of airstrikes against the Islamic State in Syria, after Prime Minister David Cameron asserted that bombing the "medieval monsters" in their heartland would make Britain safer.

The 397-223 vote in the House of Commons means Royal Air Force fighter jets - already operating against ISIS in Iraq from a base in Cyprus - could be flying over Syria within hours. Foreign Secretary Philip Hammond told Channel 4 news that the strikes would begin "very quickly . . . probably not tonight, but it could be tomorrow night."

Hours later, four Royal Air Force Tornados struck targets in Syria, but no details were given.

Antiwar demonstrators outside Parliament booed as they learned the result of the vote. The decision came after an emotional 101/2-hour debate in which Cameron said that Britain must strike the extremists in their heartland and not "sit back and wait for them to attack us."

Opponents argued that Britain's entry into Syria's crowded airspace would make little difference and said Cameron's plan was based on wishful thinking that overlooked the messy reality of the Syrian civil war.

Cameron has long wanted to target ISIS in Syria but had been unsure of getting majority support in the House of Commons until now. He suffered an embarrassing defeat in 2013 when lawmakers rejected a motion backing attacks on the forces of Syrian President Bashar al-Assad.

The mood changed after the Nov. 13 Paris attacks that killed 130 people. Both France and the United States have urged Britain to join their air campaign in Syria, and Cameron said Britain should not let its allies down.

He said that Britain was already a top target for ISIS attacks and that airstrikes would reduce the group's ability to plan more carnage.

Cameron said that attacking ISIS was not anti-Muslim but "a defense of Islam" against "women-raping, Muslim-murdering, medieval monsters."

He was backed by most members of his Conservative Party - which holds 330 of 650 Commons seats - as well as members of the smaller Liberal Democrat Party and others.

Labor was divided. Party leader Jeremy Corbyn - who represents the left wing - spoke against what he called a "half-baked intervention." But more than 60 Labor lawmakers, including senior party figures, voted in support of airstrikes.