The opening convention session of the world's largest postal union almost blew apart today as militant, pro-strike delegates erupted in a 77-minute ear-splitting demonstration aimed at blocking business until the convention voted to strike, and some of its top officers resigned.

It took more than an hour for Emmet Andrews, president of the American Postal Workers Union, to restore order after he was introduced to the 3,000 delegates who represent 220,000 union members. Andrews and other leaders who signed a tentative wage-fringe benefit settlement with the U.S. Postal Service were the targets of the chanting, aisle-blocking outburst.

In a roll call vote that took most of the afternoon, delegates voted by about a 5-to-1 margin to reject the contract. Their action is not binding on members who will decide in a mail ballot whether to accept the contract.

Reporters estimated that fewer than a quarter of the delegates in the Denver Convention Center took part in the rally. It was spearheaded by the always-militant New York Metro Union - representing about 10 percent of the union's total membership - and by Pittsburgh area locals.

But signs from Maryland, Texas, Tennessee and other areas were seen in the anticontract outburst. Preprinted placards calling the new three-year contract a "sellout," and calling union leaders strong names, were thrust in front of national officers as the crowd marched by the podium. Demonstrators demanded that officers who support the multi-billion dollar agreement resign, and that the convention be ended after taking a strike vote.

The demonstration erupted after Andrews said, "I am not a quitter" and asked the convention to return to business. Finally, a group of about 100 persons clustered around a New York Metro banner. They remained on their feet, hitting tin cans together and chanting for resignations and contract rejection while Andrews announced business meetings later in the day, and proposed that the convention take a vote whether to accept or reject the contract.

Under the union rules, the vote by delegates is merely a formality although it is an important test of sentiment. Each individual member of the AFL-CIO union, representing clerks, maintenance workers, drivers, and special delivery messengers, gets a vote to be cast by mail. Mail ballots already have started going out from the union headquarters in Washington. Earlier, the elected executive council of the union voted overwhelmingly to accept the contract. An appointed rank-and-file committee voted just as overwhelmingly to reject. A Pittsburgh union leader failed to get a court order to block the election and prevent members from voting at all.

The contract would give postal workers pay and cost-of-living raises of about 19.5 percent over three years. It retains the "no layoff" clause which would prevent the financially hard-pressed postal service from firing anyone because of economy or automation. But many members were disappointed by the pay proposals of the contract. And AFL-CIO president George Meany poured fuel on the fire by telling a national press conference that it wasn't a very good contract.

Meany's statement infuriated Andrews, Letter Carriers President J. Joseph Vacca, and James La Penta, chief negotiator from the Mail Handlerrs Union. They negotiated the contract for their 500,000 members.

All three called Meany, warning that he had undercut them and jeopardized ratification of the contract. Meany later sent each a telegram saying he spokes only about the money portion of the contract, and hailed the negotiators for retaining the pledge which gives employes - who now earn about $17,000 a year - lifetime job tenure.

Union leaders believe the majority of the nation's postal workers will vote to accept the contract, if they can get the ballots to them. There are legal moves afoot to block the voting or, failing that, to forbid a count of the ballots.

Amnesty for strikers who were fired recently after wildcat walkouts is apparently being quietly discussed by union leaders and postal officials. Strikes against the government are punishable by dismissal, a fine of $1,000 and a year and a day in jail.

Union leaders don't want to build up the hopes of members by saying amnesty is possible. Postal officials fear that if they grant amnesty now it would amount to a green light for other workers to strike.

A major postal strike could plunge the nation into paper chaos as Social Security checks, welfare funds, federal pay and pension checks and other vital letters were blocked from delivery, forcing the Army and supervisory personnel to handle the load.