Aleksei Maresyev, a Soviet fighter pilot during World War II who was acclaimed as a hero when he lost the lower part of both legs and then returned to combat, died today, just before a celebration to mark his 85th birthday, aides said.

Mr. Maresyev was taken to a Moscow hospital this morning with heart troubles, Russian news agencies reported. Hundreds of supporters had gathered in the Central Russian Army Theater for a birthday tribute when the announcer told them the news. His birthday is Sunday.

Elderly veterans wept at the news. The Russian Armed Forces orchestra, which was onstage, stood for a moment of silence.

Mr. Maresyev served in several posts in the Soviet Army in World War II. He was shot down over German-occupied territory in Russia in April 1942. Bleeding from several wounds, it took him 18 days of creeping through forests to reach safety.