DAMASCUS, Syria (AP) - The presidents of Syria and Lebanon announced Monday that Syrian forces will pull back to Lebanon's eastern Bekaa Valley by March 31, but a complete troop withdrawal will be deferred until after later negotiations.

Later, Syrian military vehicles and personnel were seen moving east in the first signs of a pullback. Syrian troops in the region had stayed put for days before Monday's movement.

The announcement, made after a meeting between Syrian President Bashar Assad and Lebanese President Emile Lahoud, said Syria's 14,000 troops will pull back from northern and central Lebanon to the east, near Syria's border.

Then, military officials from both countries will decide within a month how many Syrian troops will remain in the Bekaa Valley and how long they will stay there. The soldiers have been in Lebanon for 29 years.

After a negotiated timeframe, the two governments will "agree to complete the withdrawal of the remaining forces," the statement said.

In Beirut, at least 70,000 people - some estimates said the number was at least twice as high - gathered at central Martyrs' Square to demand that Syria leave, much larger than the demonstrations last week that led to the toppling of Lebanon's pro-Syrian government.

The agreement did not set a specific timetable for that complete withdrawal, which could fall short of demands by the United States, Israel, France, Russia and other nations that Syria completely pull its troops from its eastern neighbor.

The announcement stated, "The Syrian and Lebanese agree on continuing the withdrawal of Syrian Arab forces."

It added that the redeployment to the Bekaa Valley was in line with the 1989 Arab-brokered Taif Accord, which called for Syria to move its troops to the Lebanese border and for both countries to then negotiate the withdrawal.

A White House spokesman denounced the move as a "half measure."

"We stand with the Lebanese people, and the Lebanese people, I think, are speaking very clearly," spokesman Scott McClellan said. "They want a future that is sovereign, independent and free from outside influence and intimidation."

The United States has called for a complete withdrawal of Syrian soldiers and intelligence agents before Lebanese parliamentary elections scheduled for May. State Department deputy spokesman Adam Ereli reiterated that demand Monday.

"Until Syrian forces and intelligence agents are out of Lebanon, (U.N. Security Council Resolution) 1559 is not respected," Ereli said.

The September U.N. resolution, drafted by the United States and France, called on Syria to withdraw forces from Lebanon, stop influencing politics in the country and allow Lebanon to hold presidential elections as scheduled.

At the Beirut demonstration, young and old shouted, "Freedom! Sovereignty! Independence!" and rallied under a sea of Lebanon's red-and-white flags. They repeatedly sang Lebanon's national anthem, and some carried placards imprinted with a crescent and a cross to show unity among Lebanon's religious sects.

One group raised a banner that read, "Today we have one target: To liberate our land."

Monday's demonstration marked three weeks since the Feb. 14 assassination of former Lebanese premier Rafik Hariri triggered the peaceful campaign against Syrian control. The crowd later marched to the site of the bomb blast that killed 17 others.

Anti-Syrian sentiment in Lebanon has soared since Hariri's death, which many Lebanese have blamed on Damascus and their own country's pro-Syrian government. Both governments deny such claims.

In Washington, the Syrian ambassador to the United States, Imad Moustapha, told CNN that Syria will withdraw from Lebanon in two stages over a few months.

"We entered Lebanon to end a bloody civil war," Moustapha said. "Now we are withdrawing in compliance with international law. We are giving a good example to the rest of the Middle East."

On Monday, journalists saw two Syrian military trucks loaded with furniture heading east up the Lebanese mountains. A jeep carrying a general also drove from the Ain Dara area through an intersection on the Beirut-Damascus highway and climbed the road toward the Dahr El-Baidar mountain pass leading to the Bekaa Valley.

Seven empty trucks came down the mountain to the Syrian positions, apparently to load supplies. Two battle tanks were parked on the side of the road, pointed eastward.

Two more trucks loaded with supplies headed up the mountain. The soldiers inside each truck smiled and waved at journalists.

This area of the central mountains, known as Mdeirej, is the last position the Syrians will withdraw to in central Lebanon, according to a 1989 Arab-brokered accord and Monday's announcement. The area, about 18 miles east of Beirut, has a commanding view of Lebanon's Mediterranean coastline and controls the entrance to Dahr el-Baidar.

Assad insisted his decision to pull back his forces - first announced Saturday in a speech to parliament - did not result from mounting international pressure. Instead, Assad said his plan would put Syria in full compliance with international agreements and U.N. demands.

Syria has had troops in Lebanon since 1976, when they were sent as peacekeepers during that country's 1975-1990 civil war. When the war ended, the troops remained while Syria dominated Lebanon's politics.

During Monday's meeting, Lahoud thanked Assad for Syria's role in helping prevent Lebanon's partition after the civil war and expressed "appreciation for the sacrifices made by the Syrian army in Lebanon," Lebanese presidential spokesman Rafik Shalala said.

Lahoud stressed "the unity of the Lebanese-Syrian position in confronting the challenges" and on continuing cooperation, Shalala said.

Syria's military presence and the country's support of Lebanon's anti-Israeli Shiite Muslim militant group Hezbollah has angered both the United States and Israel.

Hezbollah's powerful leader, Sheik Hassan Nasrallah, called for a peaceful demonstration in Beirut on Tuesday to show loyalty to Damascus.