Mayor Marion Barry for the second time this week has turned down an invitation to take part in a live, prime-time television debate with his three Democratic opponents the week before the Sept. 14 primary election, an aide to Barry said yesterday.

This time WRC-TV Channel 4 offered to broadcast a Democratic mayoral debate on Sept. 9, from 10 p.m. to 11 p.m. Patricia Roberts Harris and City Council members Charlene Drew Jarvis and John Ray, the other Democrats in the race, accepted the offer, but Barry turned it down because it conflicted with two other campaign events that were already scheduled.

Barry that evening will attend a reception for his Ward 6 volunteer workers and a dinner salute to the mayor, called "Bravo Barry!", that members of several arts groups are sponsoring at the Hyatt Regency Hotel.

"Those are things we just can't cancel," Lea Adams, Barry's campaign press secretary, said yesterday.

An executive at WRC-TV said yesterday that there was no alternate time being offered for a debate.

Earlier this week, Barry's campaign aides rejected an invitation from WJLA-TV, Channel 7, for the mayor to participate in a televised debate with the other Democrats the evening of Sept. 8.

That offer was made shortly after Harris, the mayor's chief Democratic challenger, told reporters that it would be a "tragedy" if the four candidates did not debate on television.

The mayor has agreed to appear with his Democratic opponents on a WDVM-TV Channel 9 prime-time talk show Friday, Sept. 10, to be interviewed by an anchorman and a reporter in an informal newsroom setting.

He also will appear on a taped WRC-TV Channel 4 candidates forum, to be aired Sept. 4 from 7 p.m. to 8 p.m., according to a station executive.