Federal officials said yesterday that of the 126 terrorist missions against the United States that President Reagan says were detected and thwarted last year, 23 occurred in the United States, including a suspected assassination plot against Libyan dissidents in this country.

Assistant FBI Director William M. Baker said his agency, which handled the domestic incidents, could not release details of all 23 cases since many are still under investigation.

But he said that one case investigated by the Federal Bureau of Investigation led to the State Department's expulsion last June of a Libyan diplomat, Farhat Tibar, an administrative assistant at the Libyan mission to the United Nations.

Other federal sources said Tibar, 32, was thought to have been involved in a Libyan-directed plot against Libyan dissidents in the United States. The sources said the Tibar case was related to a grand jury investigation last May in Alexandria in which 15 to 18 Libyans in Virginia, North Carolina, Michigan and Colorado were subpoenaed in connection with a suspected plot to assassinate at least three Libyan dissidents.

In addition, the FBI included on its list of foiled terrorist incidents for 1984 the case of two Libyan "students" convicted of purchasing silencer-equipped weapons from an undercover FBI agent. The "students" had asked the agent to supply "hit men" to "eliminate defectors."

Baker said the agency also foiled a plot last year by seven Sikh extremists to assassinate Indian Prime Minister Rajiv Gandhi and another Indian official during a visit to the United States last summer.

Figures provided by FBI Director William H. Webster indicate that from January through November 1985 there were 695 terrorist incidents worldwide, with about 200 directed against the United States. Nearly 2,000 people were killed as a result, including 17 Americans. An additional 122 Americans were injured. Webster has said that in recent years the United States has become the target of about 40 percent of world terrorism.

A State Department official, who asked not to be identified, refused to discuss specific foreign cases among the 126 cited by Reagan at his news conference Tuesday night, but said they involved "situations that we thought were real enough and far enough along to count. We did not include in this number any situations where we suspected a terrorist incident was planned but where the available evidence and intelligence was not strong enough to make a clear-cut case."

Both FBI officials and Robert B. Oakley, director of the State Department's office of counter-terrorism, warned that the numbers given publicly are conservative. In a September speech, Oakley said: "There are unconfirmed reports of additional incidents which may have been planned against the United States, but they foreign officials are uncertain of their validity . . . . Obviously, we cannot divulge too much about our successes and about where and why the terrorists failed."

Despite a general reluctance to discuss specific cases, officials have identified several of the foreign incidents:

*House Majority Leader James C. Wright Jr. (D-Tex.) said a guard dog at an airport in Zurich, Switzerland, uncovered a plot "aimed at the destruction of the American Embassy in Rome . . . and the death of the U.S ambassador."

*Oakley said in the September speech that thwarted attacks included an attempt to bomb the U.S. Embassy in Bogota, Colombia; unsuccessful attempts to bomb the U.S. Embassy residence in Beirut and several attempts to kidnap U.S. officials in Lebanon and Europe.

*A week ago, Belgian police announced the arrest of two Arabs suspected of planning an attack on the Brussels airport similar to the Dec. 27 attacks at the Rome and Vienna airports by Palestinian terrorists.

Federal sources said the FBI maintains an interest in about half the 6,500 Libyan nationals living in the United States -- those who are thought to be supporters of Libyan leader Muammar Qaddafi.

The sources said the FBI is especially interested in Libyans with temporary U.S. visas -- an estimated 3,200 to 3,300 people, according to Duke Austin of the Immigration and Naturalization Service. They include tourists, business travelers and 1,200 to 1,300 students, Austin said, adding that the State Department recently ordered that Libyan students be barred from studies in nuclear physics and aviation-related fields.