Tens of thousands of people filled St Peter's Square for a four-hour Syria peace vigil late today.

The move was answering Pope Francis' call for a grassroots cry for peace that was echoed by Christians and non-Christians alike in Syria and in vigils around the world.

The Vatican estimated about 100,000 took part in the Rome event.

This has made it one of the largest rallies in the West against proposed US-led military action against the Syrian regime following the 21 August chemical weapons attack near Damascus.

Pope Francis spent most of the vigil in silent prayer, but during his speech he issued a heartfelt plea for peace.

He denounced those who are "captivated by the idols of dominion and power" and destroy God's creation through war.

Vatican officials have stressed that today's event was religious, not political.

But the gathering nevertheless took on the air of an anti-war rally.

Some protesters held up Syrian flags and banners in the square reading "Don't attack Syria" and "Obama you don't have a dream, you have a nightmare."

A few rainbow "Peace" flags fluttered in the breeze.

The pope entered the square from the basilica steps, foregoing his usual high-spirited drive-through in his open car.

The 76-year-old pope held up well throughout the four hours -- lasting longer than many who by the vigil's 11pm conclusion had already gone home.

He thanked those who had stayed to the end for their company, and wished them a good night's sleep.

The peace vigil marked something of a novelty for the Vatican: Nothing of its kind has ever taken place in St Peter's Square.

Pope Francis has condemned the use of chemical weapons in Syria, but has been careful not to lay blame on any one side.

Other church officials, both at the Vatican and in dioceses, have been more pointed in their criticism of any internationalisation of the conflict.

Church officials have said US-French military strikes will only exacerbate the situation for civilians, particularly Christian minorities.

Earlier,The European Union's foreign policy chief Catherine Ashton said the EU's 28 nations agreed that available information seemed to show strong evidence that the Syrian government perpetrated a chemical attack in August.

Ms Ashton was speaking at the end of a meeting of EU foreign ministers in the Lithuanian capital Vilnius, where they discussed the situation in Syria and Europe's response.

During the meeting US Secretary of State John Kerry briefed ministers on the US' position with regard to Syria.

Tánaiste and Minister for Foreign Affairs Eamon Gilmore said the United Nations is the place for working out the response to the chemical weapons attack in Syria.

Mr Gilmore was speaking after the meeting of EU Foreign Ministers in Lithuania today.

He said the evidence available pointed to the Syrian regime being responsible for the attacks, and there had to be a response to that.

Mr Gilmore said all of the members of the UN Security Council should take their responsibilities seriously with regard to Syria.

Meanwhile, US President Barack Obama told his war-weary country that America needs to use limited military force in Syria.

He said military force was required to deter future chemical weapons attacks, but said he did not want to enter into another costly and protracted war.

"This would not be another Iraq or Afghanistan," Mr Obama said in his weekly radio and internet address, previewing arguments he will make in a nationally televised address on Tuesday.

"Any action we take would be limited, both in time and scope - designed to deter the Syrian government from gassing its own people again and degrade its ability to do so," Mr Obama said.

A week ago, Mr Obama said he felt limited strikes in Syria were needed, but added he wanted to ask Congress to authorise the use of military force.