LONDON — Prime Minister David Cameron, trying to show toughness in the face of young British Muslims’ going off to fight in Syria and Iraq, proposed legislation on Monday that would give the police the power to seize the passports of Britons suspected of having traveled abroad to fight with militant groups.

Mr. Cameron told Parliament that the government would draft laws to bar people suspected of having fought with radical Islamist groups from returning to Britain, and that the police would be given temporary powers to take passports from Britons suspected of wanting to travel abroad to fight. Police and security officials would be given enhanced powers to monitor suspects who have returned to Britain, Mr. Cameron said.

With a sharply contested general election scheduled for May, the government, a coalition between the Conservatives and the Liberal Democrats, has already faced criticism that it is eroding the civil liberties of suspects for essentially political purposes.

But the rise of the Islamic State in Iraq and Syria has caused real security worries here, made worse by a recent video that appeared to show the beheading of a kidnapped American journalist by a masked Islamist fighter with a British accent.