A dissident segment of the Libyan military launched two assassination attempts recently against Col. Muammar Qaddafi, who responded by executing dozens of officers, according to intelligence reports reaching the Reagan administration.

A first attempt at a violent coup by conservative officers is said to have occurred early last month at a presidential villa outside Tripoli, the Libyan capital. The reports, which administration officials said are considered reliable, indicated that as many as 15 officers may have been executed in retaliation.

A second attempt, reportedly an assault on a convoy in which the plotters thought Qaddafi was traveling, is said to have taken place within the past two weeks and to have resulted in the execution of at least 60 more officers accused of conspiring in the plot.

Unlike an attempted coup last May, which was led by exiled opponents who had infiltrated into Libya, these two reported efforts appear to have been carried out by conservative officers in the 73,000-member Libyan armed forces. It was not clear just how widely this apparent dissatisfaction with Qaddafi's rule had spread.

According to these reports, the attacks and executions have been kept secret inside Libya. The public hanging of two Tripoli University students last year fueled open dissent among the thousands of students forced to watch. Two persons who participated in those executions later were found slain.

Opposition to Qaddafi from a segment of the Libyan military is believed to be the outcome of a power struggle pitting comparatively moderate military officers and career civil servants against the extremist revolutionary committees, on which Qaddafi has increasingly relied since the coup attempt a year ago.

According to the intelligence reports, responsibility for patrolling the border with Tunisia has been taken over by members of the revolutionary committees.

Although the intelligence reports are said to be sketchy, the belief here is that the military officers involved may have felt that this was their last chance before being totally eclipsed by the radical committees.

Qaddafi has sought to build a military force made up of the radicals as a counter to the professional armed forces -- who are widely believed to represent the only serious threat to his rule -- saying the regular Army eventually should be disbanded.

U.S. and foreign analysts here say Qaddafi's suspicions of the professional military were manifest after an attempt by officers on his life several years ago. The Libyan leader then turned to the East Germans, who helped create his personal security force.

These guards are believed to come from outside the military and to be rotated regularly. Qaddafi appears to vary his routine and to withhold information about his planned movements for security reasons.

U.S. intelligence assessments in recent weeks are said to indicate that social turmoil in Libya is greater now than at any other time during the past two years.

A reporter for United Press International, one of several American and European news organizations invited to Libya this week, quoted unnamed western and nonaligned diplomats in Tripoli as saying the country was feeling the pinch of a U.S. economic embargo. They said Libya is producing only about a third of its OPEC quota of 1.1 million barrels of oil per day because of a shortage of spare parts for the industry. They said several Libyan Airlines Boeing 707s had been grounded because Libya had been unable to obtain U.S.-built spare parts.

Declining oil revenues have prompted shortages and cutbacks in social welfare programs, but Qaddafi is reported to have maintained spending for arms and aid to groups he has supported outside the country.

Speaking Wednesday to the American and European correspondents invited to Tripoli, Qaddafi avoided internal problems and used the occasion to warn President Reagan against interfering in Sudan, where military forces toppled president Jaafar Nimeri Saturday as he was returning from a visit to the United States.

The threat followed recent overtures by Qaddafi ostensibly aimed at improving relations with the United States. One such feeler reportedly was relayed by Turkish Prime Minister Turgut Ozal.

State Department spokesman Edward Djerejian declined on Wednesday to confirm such exchanges but said, "Our position on Libya is clear. The United States is prepared to improve relations with Libya if and only if there is a significant and lasting reversal of Qaddafi's support for international terrorism and his subversion of governments. Unfortunately, Libyan involvement in such matters continues unabated."