Lebanese lawmakers have decided to extend their term by 16 months until November next year, postponing a scheduled June parliamentary election, because of political deadlock and violence spilling over from neighbouring Syria's civil war.

The move, criticized by the US and the United Nations, provoked a protest in a main square near the parliament. Demonstrators garbed in black carried signs saying they were in mourning for the democratic process in Lebanon and some threw tomatoes at convoys of politicians driving past.

Coupled with the failure of Prime Minister-designate Tammam Salam to form a government after two months of talks, the deferral of the vote deepened a sense of dangerous drift and disarray in Lebanon at a time of economic slowdown, increasing sectarian violence and a refugee influx from Syria.

The electoral law was worked out in 2008 as a compromise, but it satisfied none of Lebanon's rival sectarian groups, and their leaders remain too divided to agree on a new formula.

Over objections from one political bloc and a small but vocal protest near the parliament, 97 out of 128 legislators voted on Friday in favour of extending the term.

"They endorsed it in the first 10 minutes," a senior political source, who took part in the vote, told Reuters.

It was the first time a parliament had decided to lengthen its own tenure since Lebanon's own 1975-1990 civil war.

Critics outside parliament as well as the Free Patriotic Movement - the only bloc to vote against the extension - said the decision violated Lebanese democracy and they accused politicians of using security concerns to delay elections.

Protesters held up signs saying, "The extension of powers is an extension of all the crises Lebanon is facing," and some formed a human wall to try to prevent politicians driving up to the entrance of parliament.

The deal was struck as fighting raged for a town just over the border in Lebanon with Lebanese Shi'ite Muslim Hezbollah militants openly involved on the side of Syrian President Bashar al-Assad against Sunni Muslim rebels seeking to overthrow him.

The fighting has provoked armed clashes in the Lebanese port city of Tripoli between sectarian groups for and against Assad.

Even before violence in Lebanon escalated sharply last week, politicians were at loggerheads over changes to the electoral law and had already put the June poll date in serious doubt.

Most political parties oppose the existing first-past-the-post style system and have been discussing a hybrid law that would introduce an element of proportional representation. But they have failed to reach consensus on how that should work.

Suleiman Franjieh, head of Lebanon's Christian Marada party, argued that an extension was the only way to prevent rising political tensions from boiling over.

"We are not extending the parliament for the sake of parliamentarians, we are extending to prevent civil war," he told the state news agency. "Those who understand realise we are delaying the election so we can postpone these problems."

Michel Aoun, head of the Free Patriotic Movement, disagreed. "They have no desire to have elections and that may be for their own calculations," he said. "What are the guarantees that we will agree on an election law in the future?"

In Washington, the State Department said the United States "strongly rejects" the decision.

"We have strongly supported elections being held on time in keeping with Lebanon's legal and constitutional requirements," spokeswoman Jen Psaki said. "We believe the Lebanese people deserve the right to exercise their democratic freedoms, to elect their leaders, and to choose who best represents them."

The United Nations said the decision was regrettable.

"While it was clearly important to ensure the continuity of institutions, it was a matter of regret that no agreement had been reached on elections," UN Special Coordinator for Lebanon Derek Plumbly said in a statement.-Reuters