Officials in the coastal Southern California city of San Diego completed the largest ballot-by-mail election ever in the United States today, scoring a record turnout but giving the city's popular mayor a surprising political defeat.

Mayor Pete Wilson supported the unique mail ballot as a way to save money, increase voter participation and speed approval of a $224 million downtown convention center complex. The turnout, 60.77 percent registered voters, broke the city record for a special election, but the convention center was rejected by 56.6 percent of those voting.

"I am very disappointed because I think temporarily the city has lost an opportunity," said Wilson, a Republican who wants to become governor. He said he would try again to get the convention center approved and still felt the mail ballot had been a good way to encourage participation in a single-issue election.

Participation in this section has been carefully watched across the country, because voter turnout has been declining steadily in most areas. The high number of voters here was especially significant because a ballot question like the convention center is usually one of the least likely to stimulate voter excitement.

"We had very few problems, just a few people who forgot to sign the return envelope" to allow a check of signatures on the voting rolls, a spokesman for Wilson said. City Council member Fred Schnaubelt, a leader of the opposition to the convention center, called the vote "one more tiny victory for the taxpayers." The mail ballot, he said, was "very successful and brought much greater participation."

Otto Bos, White's press secretary, estimated that the mail ballot would cost only $275,000, compared with an estimated $500,000 if San Diego had held the election at citywide polling places as usual. The turnout did not reach the level for hotly contested general elections, which in San Diego occasionally have brought out 63 percent of the voters, but beat the record for off-year special elections -- 55.1 percent in 1971.

City officials interviewed last week said they found it eerie to campaign for a measure while balloting was going on. Ballots were mailed to 430,211 registered voters april 20 and had to be received by midnight last night. The ballots were computer cards, almost identical to those used for absentee voters in the state. Voters punched out their choices and mailed the ballot back in a special signed envelope.

The convention center was designed to revive a rundown six-block area of adult bookstores and massage parlors. Wilson argued that the increase in convention business would generate tax revenues from tourism that would pay for the center, but admitted that it was possible the city might have to pay as much as $25 million a year for the project.

A city supervisor who opposed the convention center said he thought some people may have voted no because they did not like the mail ballot. "Some thought it violated the secret ballot," he said.

Robert O. Peterson, one of the founders of the Jack-in-the-Box restaurant chain, sued to stop the vote because he thought it violated the Constitution's secret ballot provisions. Peterson's request for an injunction to stop the election was denied, on grounds the ballots could still be invalidated after the case is finally heard in court.