Supporters of an amateur athletic arena in Coney Island are hoping their opposition to a minor-league ballpark may help to improve their own plan's chances.

During three days of hearings last week before Community Board 13, city officials detailed a plan to build a $20 million, 6,500-seat ballpark for a Mets minor-league team now in Pittsfield, Mass. The colorful park is meant to revitalize Coney Island and celebrate the beachfront setting with a press tower shaped like a lifeguard stand, luxury boxes that look like cabanas and giant steel beach umbrellas that shade the stands.

But politicians and residents at the hearings asked few questions about the stadium, and instead demanded that the city build the amateur sports arena, called Sportsplex, beside the ballpark. The city and state have budgeted $67 million for the 12,500-seat arena and its accompanying gymnasium and training rooms, but design work cannot begin without Mayor Giuliani's approval.

Proponents of Sportsplex express fear that if the design work does not begin soon, the state money may dry up. They also worry that once the baseball stadium is built, the Mets will object to any loss of parking space due to Sportsplex construction.

"Everyone can be a winner if we can turn the lights on in City Hall," said August A. Tuosto, a vice president at New York City Technical College. "To build only the ballpark would be breaking a promise to our young people."

The community board will not vote on the Mets ballpark until Dec. 6, but the board appears certain to reject the proposal. The board has no direct power to stop the stadium, but Sportsplex supporters hope that "no" votes from the board and from the Brooklyn borough president, Howard Golden, who has said he opposes the Mets stadium plan, will lead to a City Council hearing on the proposal. Sportsplex supporters hope they can then convince the Council speaker, Peter F. Vallone, to trade his approval for the Mets stadium for Mr. Giuliani's approval of Sportsplex.

Bruce Brodoff, a spokesman for the Economic Development Corporation, said the city was not opposed to Sportsplex, but still had questions about whether commercial events at the arena would cover its operating costs. Supporters of the project said they thought that the city's real fear was that building Sportsplex could delay the construction of the minor-league park. The Mets minor-league team is scheduled to begin play next year at a temporary stadium in the Parade Grounds, part of Prospect Park, and then move to Coney Island in 2001.

But some people believe that if the Sportsplex dispute delays the Coney Island baseball stadium, that temporary home in the Parade Grounds will become permanent. Livia Thompson, a Fort Greene mother of two soccer players, said that could mean many weekend soccer games would disappear from the park.

"We are afraid if it gets settled in the Parade Grounds it will never move," she said.