James LeMoyne, the United Nations envoy to Colombia, with Raul Reyes, a rebel commander, on Monday.

The breakthrough came after the rebels dropped their demand that the government cancel measures it had taken to increase its control over a huge demilitarized zone that it ceded to the guerrillas three years ago as the talks began. The rebels of the Revolutionary Armed Forces of Colombia also agreed to government demands that negotiations begin immediately to reach a final cease-fire in the decades-old war.

Raúl Reyes, a top rebel commander, said he would "accept the terms" agreed to with the government on behalf of the guerrilla force. Mr. Reyes said the deal "pleases the immense majority of Colombians."

The deal appears to be a victory for Mr. Pastrana, who won office in 1998 pledging to bring peace but has seen his efforts falter as the conflict escalated across the country. In several instances in recent years, Mr. Pastrana has returned to the talks and renewed his permission to the rebels to remain in the demilitarized zone, angering a growing numbers of Colombians who have come to view the guerrillas as unwilling to negotiate.

In this crisis, Mr. Pastrana's government apparently did not back off on any its demands.

Government tanks and troops were positioned just outside the huge rebel-occupied territory in southern Colombia, ready to retake it this evening.

But at the last minute rebel commanders, working with ambassadors to Colombia from a group of 10 nations, came up with a proposal that was acceptable to Mr. Pastrana. The president, who was in telephone contact with the diplomats from Bogotá, the Colombian capital, accepted the offer, the French ambassador, Daniel Parfait, told reporters in this hamlet in the guerrilla enclave.

"This is an important moment, an important step," said Camilo Gómez, the government's chief negotiator, who arrived here shortly after the announcement. "The vast majority of Colombians are completely convinced that a politically negotiated solution is the right way."

The tense effort by the diplomats and a special United Nations envoy, James LeMoyne, came as the army was poised to retake the Switzerland- sized territory that Mr. Pastrana had ceded to the rebels as a safe haven for the peace talks. The negotiations have been held here in Los Pozos.

The negotiations were ruptured three months ago because the rebels insisted that the government stop air force flights over the demilitarized territory and end restrictions on the travel of foreigners into the zone. The rebels were especially concerned about the overflights, which they saw as provocative efforts to identify rebel bases and pinpoint the whereabouts of commanders.

Mr. Pastrana, who has been harshly criticized for buckling under rebel demands in the last three years, said that the security restrictions and fly- overs were nonnegotiable. Officials close to the talks said Mr. Pastrana was also irritated that the rebels would not accept his word guaranteeing their safety.

The government called on the rebel army, known as the FARC, to drop its security demands, open cease-fire negotiations and declare an end to kidnappings and extortion. Those points are all in an agreement that the rebels signed in October after their killing of a former culture minister deepened Colombians' bitterness about the peace efforts.

"The position of the government and the president has always been clear," Mr. Gómez said. "It is a position that has not varied. Today the FARC has accepted what the country and the president have expressed. That is what the country had been waiting for."

Mr. Pastrana, who is barred by law from running for office again in general elections this May, surprised the country on Wednesday when he went on national television to declare the talks dead, citing rebel recalcitrance.

Although many Colombians were pleased that day with Mr. Pastrana's decision, a permanent rupture in the talks would have been a disaster for a president who has had few successes. Now Mr. Pastrana still has nearly seven months to negotiate a cease- fire agreement, which many consider essential before a final overall peace accord is signed.

Just yesterday, it appeared that the possibility of restarting the talks was dead when the rebels announced that they were withdrawing from the zone, angrily blaming Mr. Pastrana for the breakdown.

A day earlier, on Saturday, the rebels had offered a proposal to resume negotiations that was rejected by Mr. Pastrana. He then gave the FARC 48 hours, until 9:30 p.m. today, to leave the zone.

However, Mr. LeMoyne continued to meet with the rebels, urging them to come up with a proposal the government would accept. Today he was joined by the 10 ambassadors, who represent a group of nations that has been trying to help the two sides reach peace.

The accord was celebrated by many of the 90,000 people in this rebel-held zone.

Although many of them chafe under rebel rule, this region has avoided the chaotic violence of the rest of Colombia because neither the army nor an outlawed right-wing paramilitary group operate here.

Many feared that if the talks had collapsed for good, the rebels would withdraw and the paramilitaries would enter the zone to attack those they considered to be guerilla sympathizers.

Hours before the deal was struck a caravan of cars and trucks arrived here with local residents waving flags and chanting "Viva la paz," or "Long live peace."

"This is so good," said Norberto Rodríguez, 40, who held his baby daughter. "This is a solution for all Colombians."

Coming to an agreement on a cease-fire will not be easy, diplomats here said.

"I think what the parties agreed to do is to get back together to see where we were," said Guillermo Rishchynski, the Canadian ambassador, one of the diplomats here. "From that standpoint, I think we've made some progress. What happens when they sit down is still subject to the ebb and flow of negotiations."