OAS Secretary-General Jose Miguel Insulza said Honduras "risks being suspended" from the group if the leaders of the military coup do not comply.
Insulza delivered what he called "an ultimatum" as OAS talks regarding the crisis wound past the eight-hour mark.
The 72-hour period appears designed to cover plans for Zelaya, who was ousted in a coup Sunday, to go home, accompanied by the presidents of Argentina and Ecuador, and seek restoration of his authority.
Honduras' ousted president won overwhelming international support Tuesday at the United Nations as he planned the high-profile return to his chaotic country.
The politicians who sent soldiers to fly Zelaya into exile in his pajamas, however, warned Tuesday that he would be arrested for treason if he tries to return home.
The showdown was building to a climax as the presidents of Argentina and Ecuador signed on to accompany Zelaya and other figures on a flight to Honduras on Thursday. Attorney General Luis Alberto Rubi said Zelaya would be seized "as soon as he sets foot on Honduran soil" and face 20 years in prison on charges that also include abuse of authority.
"I'm going back to calm people down. I'm going to try to open a dialogue and put things in order," Zelaya said at the U.N. "When I'm back, people are going to say ... `commander, we're at your service' and the army will have to correct itself. There's no other possibility."
The U.N. General Assembly voted by acclamation to demand Zelaya's immediate restoration.
With no international support but a significant following at home, the new Honduran leadership called thousands of flag-waving people into a downtown plaza. Soldiers fenced off the area around the presidential palace, where security forces used tear gas and water cannons Monday against Zelaya supporters, injuring and arresting dozens.
The interim president named by Congress, Roberto Micheletti, said Zelaya could be arrested for violating the constitution if he returns. He also said he would not resign no matter how intense the international pressure on Honduras becomes.
"No. I was appointed by Congress, which represents the Honduran people. Nobody can make me resign unless I break the laws of the country," Micheletti said in an interview with The Associated Press at the presidential palace.
Zelaya - whose elected term ends in 2010 - had defied the Supreme Court and called a referendum on constitutional change that opponents worried would lead to Zelaya prolonging his presidency.
Zelaya backed down from the referendum on Tuesday, saying at the U.N. that he would no longer push for the constitutional changes he had wanted.
"I'm not going to hold a constitutional assembly," he said. "And if I'm offered the chance to stay in power, I won't. I'm going to serve my four years."
He said he would then go back to being a farmer - a humble description considering the wealth he has accumulated in ranching and agribusiness.
"I come from the countryside and I'm going to go back to the countryside," he said.
But Micheletti told AP that Zelaya had already violated Honduran law and it was too late for him to avoid arrest if he returns to Honduras.
Micheletti also said he was worried about the possibility of invasion from other Latin American countries, although he did not say which ones.
But earlier Tuesday, Venezuelan President Hugo Chavez, Zelaya's top ally, said any aggression toward Zelaya from Micheletti's government should prompt a military intervention by the United Nations.
"We thank God for not letting democracy be interrupted in our country," Micheletti said earlier before a crowd of some 5,000 cheering supporters in white and blue, the colors of the Honduran flag. Flanked by soldiers in camouflage, he said: "The heroes of this democratic day are the soldiers."
A pro-Zelaya crowd of about equal size marched toward the presidential palace, now protected by a chain-link fence and more than 500 soldiers and police. They dispersed as rain began to fall in the late afternoon with no reports of violence - a contrast with Monday when Zelaya said more than 150 were injured and 50 arrested. Micheletti's government didn't release figures.
Micheletti's foreign minister, Enrique Ortez, threw a wild card onto the table, telling CNN en Espanol that Zelaya had been letting drug traffickers ship U.S.-bound cocaine from Venezuela through Honduras. Ortez said the U.S. Drug Enforcement Administration was aware of Zelaya's ties to organized crime.
Spokesman Rusty Payne could neither confirm nor deny a DEA investigation.
The United States stood firmly by Zelaya, however, with State Department spokesman Ian Kelly saying Washington sees no acceptable solution other than Zelaya's return to power. He said the United States is considering cutting off its aid to Honduras, which includes $215 million over four years from the U.S.-funded Millennium Challenge Corporation.
Micheletti said he had no contact with any U.S. authorities since assuming the presidency.
The U.N. vote added to an avalanche of international denunciations of the military's removal of Zelaya on Sunday, which recalled the dark days of dictatorship for which Latin America was long notorious. The world body called on all 192 U.N. member states to recognize only Zelaya's government in Honduras.
Meanwhie, blocked trucks began lining up along Honduras' borders as neighboring countries imposed a trade ban.
Some local television stations remained off the air and local media carried few reports of demonstrations in Zelaya's favor, apparently under government pressure. Ortez said freedom of expression was in full force but did not directly address the closure of stations or the temporary detention of journalists.
The U.S. military, which has close ties to Honduran commanders, tried to avoid getting caught up in the dispute. It ordered most of its 800 personnel to remain inside the Soto Cano air base, 60 miles north of Tegucigalpa, allowing only "mission-essential" tasks, Southern Command spokesman Jose Ruiz said in Miami.
Honduras receives about $1 million a year from the United States to fight drug trafficking and Soto Cano is a key base in the fight against drugs. There's also a contingent of DEA agents in country. U.S. and Honduran officials estimate about 100 tons of Colombian cocaine pass through Honduras annually on their way to the United States.