BEIRUT, Lebanon - Lebanon's feuding pro- and anti-Syrian factions looked set for a compromise after an opposition rally drew protestors on to the streets of Beirut in numbers the army said were "unprecedented."

An Arab League envoy was due in Beirut Monday after announcing he had received a positive response from the pro-Syrian opposition to proposals to end a political crisis which has paralysed the government and raised fears of a return to civil strife.

The envoy, Mustafa Ismail of Sudan, told Dubai-based Al-Arabiya television that he had received the "agreement in principle" of Shiite militant group Hezbollah, which has been spearheading the 10-day-old opposition protests, and was returning to Beirut for further talks.

The opposition has been demanding that the Western-backed cabinet of Prime Minister Fuad Siniora make way for a government of national unity. Siniora has accused it of trying to mount a "coup."

Ismail said what still needed to be thrashed out was the details of the opposition's participation in the government after its six ministers pulled out last month.

"The opposition says I do not want to bring down the government or block its work, and I can give the required guarantees," Ismail said.

"The government in turn says I welcome the opposition's participation ... and am ready to give guarantees on this score. Hence, what we need is additional (discussion of) details of these guarantees."

Hezbollah MP Hassan Fadlallah confirmed that the movement's leader Hassan Nasrallah had given a positive response to the Arab envoy.

"Nasrallah has informed Mustafa Ismail that Hezbollah sees positively any initiative that includes the formation of a government of national unity which secures a blocking minority," Fadlallah said.

"But in the end our position will be decided after being discussed among opposition leaders," he added.

Earlier in the day, cries of "Siniora, out!" echoed across Beirut as hundreds of thousands of anti-government protesters packed into the heart of the Lebanese capital.

"Change is coming," the opposition banners boasted as a sea of demonstrators waving Lebanese flags spilled into the streets surrounding the prime minister's office.

But in evidence of continuing deep division in the country, pro-government supporters rallying in the main northern city of Tripoli also brought out a vast crowd, which organisers estimated to be in the hundreds of thousands.

In Beirut, where some 20,000 troops deployed on the streets, an army spokesman estimated that "hundreds of thousands" had gathered.

"This is a sea of demonstrators unprecedented in the history of Lebanon," the spokesman said.

Government supporters have staged large counter-rallies amid the deadlock that threatens to paralyze the legislative process with the speakership of parliament in the hands of the opposition.

The opposition accuses the government of weakness and corruption, and says it no longer represents the people after the six pro-Damascus ministers submitted their resignations last month.

The protesters want to replace the current cabinet, formed after 2005 elections, with a national unity government that they say is required by the power-sharing arrangements in force since the end of the 1975-90 civil war.

Siniora, who has received messages of support from Britain, France, Germany, the United States, and some pro-Western Arab states, has pledged that what he branded an attempted "coup" by Hezbollah will fail.

Siniora's coalition has accused the opposition of seeking to block cabinet endorsement of plans for an international tribunal to try suspects in the 2005 murder of former premier Rafiq Hariri, widely blamed on Syria.

Damascus has strongly denied any involvement.

Pro-Syrian President Emile Lahoud on Saturday formally rejected the cabinet decision on the court, saying it should be "be reviewed by a legal, constitutional and consensual government." -AFP