Two Filipino dissidents who escaped from a Manila prison three weeks ago urged the United States yesterday to end military assistance to the government of President Ferninand Marcos.

Sergio Osmena III and Eugenio Lopez Jr. are in Washington to thank members of Congress who took a personal interest in their case during their five years in detention.

At a news conference both denied a government charge that they had plotted to assassinate Marcos.

Osmena is the son of the defeated liberal candidate for president against Marcos in the Philippines' last election in 1969. Lopez was publisher of the Manila Chronicle, an anti-Marcos daily that was shut down when martial law was imposed in 1972.

Osmena said a U.S. decision to suspend military aid would have a "tremendous psychological effect" on the Philippines and could prompt internal reforms.

Alternatively, he said, when the Philippine people "see the U.S. giving open approval, it is a big blow." Osmena, 33, and Lopez, 48, staged their escape from Manila's Fort Bonifacio stockade a week after U.S. Assistant Secretary of State Richard Holbrooke visited the Philippines to lay the groundwork for a resumption of talks on a new agreement governming U.S. military bases there.

Osmena said he and Lopez suffered no physical torture during their detention but they were subjected to substantial "mental torture," usually taking the form of extended periods of solitary confincment.

He described the psychological pressures as "subtle but painful and effective."