The anti-Syrian opposition, seeking to regain its momentum, urged supporters to hold a massive demonstration in Beirut (search) Monday to demand a full Syrian withdrawal, the resignation of Lebanese security chiefs and the truth behind the killing of a former prime minister.

Syrian military intelligence, meanwhile, vacated another office.

The planned anti-Syrian protest — muscle flexing to counter pro-Syrian and pro-government demonstrations — comes exactly four weeks after the assassination of Prime Minister Rafik Hariri (search). The protest is also an occasion for people to vent their anger at the reinstatement of Prime Minister Omar Karami (search), who was forced to resign Feb. 28 under popular opposition pressure.

Most Syrian intelligence offices — the widely resented arm through which Syria controls many aspects of Lebanese life — remained staffed in northern and central Lebanon, though Syrian troops have redeployed closer to the Syrian border. About 4,000 of the 14,000 troops in Lebanon have left altogether.

Early Monday, Syrian intelligence officers packed up and left the town of Amyoun in the Koura region of northern Lebanon, local residents said. Syrian intelligence left offices in the towns of Aley and Bhamdoun in the central mountains east of Beirut last week.

The planned protest came amid a flurry of diplomatic activity. A day after meeting with Syrian President Bashar Assad, U.N. envoy Terje Roed-Larsen held back-to-back meetings Sunday with top Lebanese government and opposition officials to ensure the implementation of a Security Council resolution demanding Syria's withdrawal.

Roed-Larsen has indicated he extracted further details of a pullout timetable from Assad during their meeting Saturday. He left the region Monday and was heading for U.N. headquarters in New York.

A senior Lebanese army officer said that 4,000 Syrian soldiers — more than a quarter of those serving in Lebanon just a week ago — are back in Syria, but he said a complete withdrawal is at least a month away.

And a Syrian Cabinet minister, Buthaina Shaaba, also indicated in an interview with CNN that a "very fast timetable for withdrawal" was expected — with completion ahead of Lebanese parliamentary elections as President Bush has demanded. Election dates have not yet been set, but the current parliament's mandate expires May 31.

Calls for protest were made in every town and village in the country Sunday, urging people to descend on Beirut to demand "the truth" of who killed Hariri and 17 others in the bombing on a Beirut street.

Many answering the protest calls are particularly offended by the reinstatement of Karami, who resigned amid opposition protests Feb. 28 but was brought back 10 days later by President Emile Lahoud 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 organized by the Shiite Muslim militant group Hezbollah in which hundreds of thousands of pro-Syrian supporters participated.

As part of the tit-for-tat protests, at least 100,000 pro-Syrian demonstrators turned out Sunday in another Hezbollah-organized protest in the southern market town of Nabatiyeh, burning Israeli flags and waving posters of Assad, his late father, President Hafez Assad, and Lahoud. The group which fought Israeli forces in southern Lebanon is considered terrorist by the United States.

Demonstrators shouted "Death to America" and "Death to Israel" and slogans denouncing U.N. Resolution 1559, which was drafted by the United States and France last year calling for Syria to withdraw its then 14,000 troops from Lebanon and stop interfering in internal Lebanese affairs.

In tightly controlled Syria, there were protests, too — in favor of the government. Syria's official news agency claimed hundreds of thousands of flag-waving Syrians rallied in the northwestern city of Lattakia Sunday in support of Assad.

In a smaller Beirut counter protest, thousands of anti-Syrian demonstrators held candles high, with the flames spelling out the word "truth."

Syria has been Lebanon's main power broker for nearly three decades since sending troops to its smaller neighbor in 1976 to help quell what was then a year-old civil war. The troops, at times numbering more than 35,000, remained after the war ended in 1990.