ASUNCION, Paraguay (AP) _ Troops and armored personnel carriers swept into downtown Asuncion on Friday as hundreds of demonstrators gathered outside Congress to demand for the ouster of President Raul Cubas.

Paraguayan television and radio reported up to 15 armored vehicles sped through the capital, many barreling through red lights.

Shortly after the vehicles were dispatched, Walter Bower, president of Paraguay's lower house, pleaded with the military to refrain from intervening in the country's political turmoil. He also promised that the Senate's impeachment trial for Cubas, scheduled for Saturday, would move forward.

In the plaza adjoining Congress, Francisco Oliva, a Jesuit priest, said Mass, beseeching God to aid the country in ``a difficult time.''

Earlier in the evening, protesters demanding the ouster of Cubas drove riot police from the plaza after a pitched battle of flying rocks, tear gas, and water cannons.

The half-hour melee was the biggest outbreak of violence in a four-day crisis that has paralyzed this Latin American country. Business came to a standstill as many residents stayed in their homes to monitor developments on radio and television.

Cubas was impeached by Paraguay's lower house on Wednesday, accused of abuse of power by illegally freeing a controversial army general from jail.

Cubas' lawyers are set to defend him when a scheduled Senate trial gets under way on Saturday. Lawmakers said the trial would continue through next week.

If convicted by at least two-thirds of the 45-member Senate, Cubas would be stripped of power and replaced by Senate President Luis Gonzalez Macchi.

There was no immediate report of serious injuries from the clashes between police and demonstrators in the broad plaza outside Congress.

After retreating earlier in the day, hundreds of demonstrators surged back into the square, shouting ``Cubas resign!'' and shooting off fireworks. Order was later restored.

Tuesday's ambush slaying of Vice President Luis Maria Argana accelerated efforts to impeach Cubas. Many of Cubas' opponents in Congress blame him for the political turmoil that preceded the shooting.

For months they have protested his release of controversial former Gen. Lino Oviedo. The general, perceived by many as the strongman behind Cubas' presidency, was serving a 10-year sentence for attempting to oust then-President Juan Carlos Wasmosy in April 1996 when he was freed in August by the newly-inaugurated Cubas.

When the Supreme Court ordered Cubas in December to send Oviedo back to prison, the president refused, despite pressure from opponents in his own ruling Colorado Party.

There have been outbreaks of violence almost every night since Tuesday. A clash between demonstrators and police Thursday left several people slightly injured.

Francisco Alonzo, manning the counter at the Alba hardware store, said business had dropped 80 percent since Tuesday.

``Everyone coming in here is asking for locks and deadbolts,'' he said, shrugging as he looked over an array of security devices. ``They're concerned.''