Five political leaders of El Salvador's leftist revolutionary movement were seized today and at least four of them reportedly killed by a right-wing death squad allegedly working in cooperation with elements of the government's security forces.

The action is a major blow not only to this country's revolutionary organizations but to whatever hopes have developed in recent weeks for a negotiated end to the brutal fighting here.

Leaders of the Revolutionary Democratic Front and the Roman Catholic Church said front President Enrique Alvarez and Juan Chacon, who is secretary general of the Popular Revolutionary Bloc, were among those secretary general of the Popular Revolutionary Bloc, were among those picked up this morning at the church's legal aid office here in the capital.

The others arrested were Manuel Franco of the communist National Democratic Union, Humberto Mendoza of the Popular Liberation Movement, and Enrique Barrera of the social democratic National Revolutionary Movement. Doroteo Hernandez, who was apparently seeking aid in the offices, also was detained.

The front is the political umbrella organization for the broad-based leftist coalition that has been trying to overthrow the U.S.-backed civilian military junta here. All of those arrested except Hernandez are members of the front's leadership directorate.

Chacon's revolutionary bloc is one of its largest member groups, itself composed of peasant, union and professional associations. Alverez and Chacon have traveled extensively to promote their cause in Latin America and Europe, where the front maintains more than a dozen offices.

The church said the operation was carried out by a large group of heavily armed men in civilian clothes accompanied by others in the uniforms of the National Police and Army commandos.

They burst into the church offices in the San Jose Secondary School, forced everyone to lie face down, and took away the six prisoners.

The government has thus far denied any participation or knowledge of the action beyond what has been reported publicly.

Four bullet-riddled bodies were discovered late this afternoon near Lake Llopango, about 12 miles from here, and taken to a local funeral home. there, local reporters identified the bodies of Chacon, Mendoza, Barrera and Hernandez.

Subsequently a man identifying himself as a member of an anticommunist group called Comando Maximilano Hernando Martinez called a local radio station and claimed responsibility for the killings. There was no indication of the fate of Alvarez and Franco.

In a statement two weeks ago, the commando announced its intentions to eliminate communist thieves and prostitutes."

U.S. Ambassador Robert White, an embassy spokesman said, has expressed his "grave concern" about the events to the junta.

There are a number of serious worries from the embassy's point of view.

From the standpoint of those who have advocated a negotiated settlement to the near-civil war here, which has cost an estimated 9,000 lives, this year, the seizure of the left's political leadership could not have come at a worse time.

In recent weeks there was growing, if still faint, hope here that the Revolutionary Democratic Front could be brought to the bargaining table. It appeared to be losing political momentum both domestically and internationally and its guerrilla allies were suffering several setbacks militarily.

At the same time, however, rightists opposed to the government's more liberal elements and heartened by Ronald Reagan's victory in the United States have pushed with new vigor to force changes in the Salvadoran government that most likely would preclude any negotiation with the left, no matter how much it is weakened.

There is also concern among the government's moderate elements and U.S. diplomats that an extremely conservative government would actually strengthen the left by forcing new recruits into its ranks and enhancing the revolutionaries' image abroad.

The balance of forces, already extremely delicate, is thrown completely out of kilter by the seizure of the leftist leaders. The end result, moderates fear, could be the all-out civil war on which El Salvador has been verging for over a year.

Both the extreme right and the extreme left have sought to polarize the nation.

Today's action, coming only hours after the government made public an alleged leftist plot to assassinate 80 officials -- including the junta -- marks what one observer called "a clear escalation of the conflict."