Policemen tonight rejected the latest contract offer designed to end their 15-day-old walkout.

Their action marked a sharp turnaround from the optimism expressed earlier today by union leaders, who order their men in the teamsters affiliated local to return to work, and by the police chief, who had said the strike was virtually over.

The unofficial vote was 447 to 173 against the offer.

The question of amnesty for the striders was the basic issue. And the possibility of a favorable vote by the police apparently foundered over the details of who would be eligible. Some of the officers had been involved in minor violence during the strike in support of their position.

The effect the vote will have on the police force is unclear, although 900 of the 1,514-member force reportedly had returned to work after the leaders' order.

The strike had already sabotaged this year's Mardi Gras celebration.

Before tonight's vote, Teamster official Mitchel Ledet called the proposal "an agreement that I can recommend." And Police Superintendent James C. Parsons said, "For all intents and purposes the strike is over. Your police department is back on the street."

The union had been scheduled to vote on a contract Friday night, but union leaders postponed that action, saying that the city would have another offer. Mayor Ernest N. Morial denied that report, and James Chubbuck, one of his executive assistants, said, "We've reached the point where the city has come about as far as it can go."

No details of the documents that the Teamsters were to consider were released before the vote. But what Morial had called his "final final" offer included amnesty for strikers, more liberal computation of overtime and holiday pay and better hospitalization and insurance policies.

However, the city refused to let ranking officers be covered by a labor contract. This had been a key demand by the police union.

Before the vote, Chubbuck said that if the contract is rejected, "the city will have to completely reassess its position." Such an action could include reevaluation of all offers made to the union, he said, and he did not rule out the possibility that the city might refuse to recognize the union as the policemen's sole bargaining agent.

Recognition was the principal issue in an earlier police strike that began Feb. 8. That walkout ended in 42 hours when policemen agreed to return to work for a week on an interimbasis while other points of dispute were worked out.

On Feb. 16, when no resolution seemed imminent, the second strike began.

Within hours of the strike vote Morial canceled the first three days of Carnival parades -- Feb. 17 through 19 -- in New Orleans. On Feb. 20, when settlement of the walkout appeared unlikely before Mardi Gras (Feb. 27), leaders of 18 parade-giving organizations called off their processions in New Orleans. Some groups, however, got permits to march in suburban parishes (counties).

In another union development the city's firefighters were supposed to complete voting on their new contract Friday night, but Clarence J. Perez, their union president, canceled that session because he feared violence by people not in that union -- including, perhaps, striking policemen.