Security was tightened in the Wall Street financial district yesterday after the Puerto Rican terrorist group FALN exploded four powerful bombs to protest "Yankee imperialism."

The New York and American stock exchanges and two other buildings were damaged late Sunday.

The blasts came less than an hour before the 28th anniversary of the March 1, 1954, attack by Puerto Rican nationalists on the House of Representatives in which five congressmen were wounded by gunfire.

The FALN claimed responsibility for the explosions that damaged the stock exchanges and the headquarters of Merrill Lynch and the Chase Manhattan Bank just before midnight.

There were no injuries, but streets and plazas were littered with shards of glass.

As executives, brokers, clerks and secretaries began filling Wall Street on their way to work yesterday, guards checked identification cards, briefcases and packages at the two exchanges.

Since 1974, the FALN has claimed responsibility for about 120 bombings in the New York City area, Chicago and Washington. Five people died in the attacks, four of them when a bomb went off at New York's historic Fraunces Tavern in January, 1975. Last May, the FALN planted five bombs around the city, including the U.S. Mission to the United Nations.