Ignoring a show of force by hundreds of riot police, 3,000 Polish farmers prayed and sang yesterday at a mass marking the first birthday of the independent Rural Solidarity union, now suspended under martial law.

The mass ended without incident although the atmosphere remained tense as riot police made their second appearance on Warsaw streets in two days. Political and labor demonstrations are illegal under the Dec. 13 martial law decree, but religious gatherings are permitted.

"Let us pray to God for the revival of Rural Solidarity to uplift life in the countryside and improve Polish agriculture," Warsaw Bishop Wladyslaw Miziolek said during the mass at St. John's Cathedral.

Industrial Solidarity's leaders have called for 15-minute strikes today, which marks five months of martial law.

About 3,000 peasants, many wearing Rural Solidarity pins, jammed the cathedral for the mass or stood in the cobblestone streets.

Busloads of police backed by water cannon arrived at the nearby Victory Square parade ground about 30 minutes before the mass and took up positions at several points around the cathedral.

The display of police power underscored the government's concern about a recurrence of rioting that erupted in the old town May 3 during an unauthorized Solidarity rally.