Flag-waving crowds from across Lebanon gathered in Martyrs'
Square in central Beirut on Monday, just metres away from al-Hariri's grave, to demand an international inquiry into his killing, the sacking of Syrian-backed security chiefs and a total Syrian pullout.
The opposition rally came a day after huge crowds turned out in the south for an anti-US demonstration organised by Lebanon's Shia Muslim Hizb Allah group, an ally of Syria.
Organisers of the Beirut protest say it will draw hundreds of thousands to the central Beirut square that has been seeing daily protests demanding a full Syrian withdrawal from Lebanon.
It could be the last of a series of demonstrations used by each side of Lebanon's political divide to show their strength.
The rally will provide an opportunity for people to vent their anger at the reinstatement of Prime Minister Umar Karami, who was forced to resign on 28 February under popular opposition pressure.
Political sources said fears were growing that protests and rallies, though peaceful so far, could turn violent amid deep political divisions over Syria's role in Lebanon that have surfaced since al-Hariri's death.
Intelligence office vacated
Also on Monday, Syrian military intelligence vacated one more office.
Most of the offices of Syrian military intelligence - the widely resented arm through which Syria controls many aspects of Lebanese life - have remained in northern and central Lebanon after Syrian troops began redeploying east closer to the Syrian border last week, with some already back home.
Early on Monday, Syrian intelligence officers packed up and left the town of Amyun in the Qura region of northern Lebanon, local residents said.
Syrian intelligence left offices in the towns of Alay and Bhamdun in the central mountains east of Beirut last week and headed to the Syrian-controlled areas of eastern Lebanon.
Meanwhile, there has been a flurry of diplomatic activity. A day after meeting Syrian President Bashar al-Asad, UN envoy Terje Roed-Larsen held back-to-back meetings on Sunday with top Lebanese government and opposition officials to ensure the implementation of Security Council Resolution 1559, demanding Syria's withdrawal.
A senior Lebanese army officer said on Sunday that 4000 Syrian soldiers - more than a quarter of those serving in Lebanon just a week ago - were back in Syria, but he said a complete withdrawal was nearly a month away at the earliest.
About 4000 Syrian troops left
Lebanon to Syria on Sunday
And a Syrian cabinet minister, Buthaina Shaban, also indicated a fast timetable for withdrawal was expected - with completion before Lebanese parliamentary elections as US President George Bush has demanded.
Election dates have not yet been set, but the current parliament's mandate expires on 31 May.
Calls for protest
Calls for a protest were made in every town and village in the country on Sunday, urging people to descend on Beirut on Monday to demand the truth about who killed al-Hariri along with 17 others in the bombing on a Beirut seafront street.
Many people answering the calls have been particularly offended by the reinstatement of Karami, who resigned amid opposition protests on 28 February but was brought back to office 10 days later by President Emile Lahud after the pro-Syrian, pro-government camp flexed its muscles.
Karami last week said his supporters had the majority in parliament and with the people, a reference to last Tuesday's "Thank you Syria" rally organised by the Shia Muslim group Hizb Allah in which hundreds of thousands participated.
Emile Lahud met Roed-Larsen
"It was a massive demonstration that asserted our legitimacy in the Lebanese street," Karami said on 10 March in accepting the invitation to form the next government.
So people were urged by email and mobile phone messages to turn out on Monday to prove Karami wrong, show loyalty to al-Hariri and to Lebanon.
Meanwhile, the Syrian delegation decided to walk out from a meeting of the Euro-Mediterranean parliaments being held in Cairo on Monday.
The walkout came in protest over not including an article about the withdrawal of Israel from the occupied Syrian Golan Heights within a draft resolution that would be released at the end of the meeting.
Aljazeera's correspondent in Cairo reported that the walkout came after Egyptian-Arab attempts of mediation failed as officials at the political committee refused to include the clause, saying the Syrian demand had come too late.