President Reagan provided $20 million in emergency military assistance to Honduras yesterday to help repel what administration officials said were attacks across the border by 1,500 Nicaraguan troops aimed at destroying a training center of the anti-Sandinista rebels.

The Honduran government, in a statement issued by its embassy here, confirmed the large-scale "incursions." White House chief of staff Donald T. Regan said earlier in the day that U.S. pilots and helicopters already in Honduras for military exercises would be used to transport Honduran troops to the border area, but later reports from Honduras made it appear unlikely that such assistance would be requested.

The Nicaraguan Embassy denied an invasion had taken place and said the Reagan administration was engaged in a "cheap maneuver" designed to win approval of its pending $100 million aid package for the rebels, known as contras.

The Sandinista attack was viewed by White House officials as bolstering political support for the aid request, which was rejected by the House last week but is expected to pass the Senate with some conditions attached. The attack was viewed with consternation and anger by House Democratic leaders who had opposed the aid request.

"There's no question that this adds urgency and impetus to our side," said White House political assistant Mitchell Daniels. "The landscape has changed dramatically," said Dennis Thomas, deputy to chief of staff Regan. The action "confirms the doubts and suspicions many people had" about the Sandinistas, he said.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) reacted by calling Nicaraguan President Daniel Ortega "a bumbling, incompetent, Marxist-Leninist communist," and said the incursion would cause some Democrats to change their votes and support the contra aid package. Sen. Patrick J. Leahy (D-Vt.), vice chairman of the Senate Select Committee on Intelligence, said sarcastically that he "had heard a rumor that Daniel Ortega is secretly on the payroll of one of our intelligence agencies as a lobbyist for the administration."

Sen. David F. Durenberger (R-Minn.), Intelligence Committee chairman and an opponent of assisting the contras, said the invasion had probably guaranteed congressional passage of the aid package, which has $70 million in military assistance and $30 million in nonlethal aid.

The $20 million provided to Honduras yesterday was formally requested by new Honduran President Jose Azcona for what White House spokesman Larry Speakes said was the "unforeseen emergency" of the Sandinista cross-border attack. The State Department said the military aid requested includes "air defense weapons, conventional ordnance, emergency spare parts and armament for helicopters and essential training."

Secretary of State George P. Shultz, traveling in Europe, said that the U.S. aid would "allow Honduras to do whatever it wants to do" in response to "the invasion by Nicaraguan communist troops," staff writer Joanne Omang reported from Athens.

While officials emphasized that the assistance was intended for Honduran military forces, the items requested are similar to those that the administration is seeking for the contras. Asked whether any of the equipment would wind up in contra hands, Speakes said, "I don't know."

Honduras received $67.4 million in military aid in fiscal 1985 and $59.8 million in fiscal 1986 after cuts required by the Gramm-Rudman-Hollings budget-balancing legislation. The request for fiscal 1987 is $88.8 million.

The $20 million that Reagan released yesterday comes from Defense Department funds previously appropriated. The formal notification of the emergency aid that the president sent to Congress said "this assistance will be in the form of defense articles in the stocks of the Department of Defense, defense services of the Department of Defense and military education and training."

Reagan complied with existing law by notifying Congress of the emergency aid, which Speakes said would be used "to repel this and future attacks." The White House spokesman said, "We have instructed U.S. commanders that U.S. personnel are not to be introduced into combat situations."

The administration version of what happened in Nicaragua was detailed by State Department spokesman Charles Redman, who said that last Saturday, "within 48 hours of rejection of aid to the Nicaraguan resistance, Sandinista military units crossed into Honduras in what appears to be a large-scale effort to locate and destroy resistance logistics bases, training centers and medical facilities which they believe to be in the area. Contrary to some reports, this does not seem to be a hot-pursuit operation by the Sandinistas, since no resistance units were withdrawing from Nicaragua at the time of the Sandinista attack."

Redman said that on Sunday morning, a large Sandinista force conducted four assaults near a center for Nicaraguan refugees situated more than 15 kilometers or nine miles north of the border in Honduras. He said these attacks "were reportedly repulsed by new resistance student volunteers which were armed that very morning."

By late Sunday evening, several Sandinista "special counterinsurgency battalions," normally accompanied by Cuban advisers, were engaged in the battle, Redman said. One of the Nicaraguan battalions, which other officials said included about half of the Sandinista troops, attempted to withdraw at this point but found their route blocked by "a large resistance column," the spokesman said.

The battle continued through Monday. Redman said the Sandinistas supported their forces with heavy artillery and rocket fire from Soviet-made launchers and helicopter gunships.

Administration officials said that it was not clear late yesterday what the military situation was along the border. Redman quoted Honduran and contra sources as saying that four Sandinista battalions of reinforcements were expected to join the attack. But other officials said that one Sandinista battalion had already withdrawn and that the other was likely to do so as soon as it could disengage from combat.

Speakes and Redman said that the contra forces had taken a number of Sandinista prisoners.

A senior administration official, asked why the Sandinistas had launched such an invasion at this sensitive time, speculated that the Nicaraguans had seized an opportunity to deal a supposedly quick and "crippling blow" to a large contra force. He said the Sandinistas apparently gambled that Honduras would not publicize the incursion, since this required acknowledgment that contra troops are based in Honduras. In fact, Honduras initially refused to confirm the incursion.

Another senior U.S. official said the Sandinista troops had tried to withdraw earlier but found themselves "pinned down, caught in a crossfire."

Reagan sent Gen. John Galvin, commander of the U.S. Southern Command, to Honduras to provide information and advice to the Honduran government and assess the situation on the ground, the White House announced.