The Israeli Defense Ministry today fired the mayor of occupied Gaza, the third such removal of an Arab mayor for Palestinian nationalist activities since Israel invaded Lebanon.

Rashid Shawa was removed from office for what the ministry called "clearly anti-Israeli activities" in dealing with the Palestine Liberation Organization.

With his departure, only Mayor Elias Freij of Bethlehem remains in power from among the top echelon of political leadership in Arab territories occupied by the Israeli Army since the 1967 Middle East war.

In Washington, the State Department expressed deep regret at Israel's dismissal of Shawa. Spokesman Dean Fischer, asked about the action, said in a statement:

["We deeply regret the dismissal of Mayor Shawa. As the appointed mayor of Gaza, both under Egyptian administration prior to 1967 and since Israel's occupation, he has been recognized as a legitimate representative and moderate spokesman for the concerns of his Palestinian constituents. Leaders with these qualities will be needed as we move toward resolution of the Palestinian issue under Camp David."]

The dismissal, which followed by three days that of mayor Shawqi Mahmoud of Jennin, underscored Defense Minister Ariel Sharon's determination to rid Gaza and the West Bank of nationalist Palestinian leaders, many of them elected, and replace them with appointed officials more amenable to cooperating with Israeli occupation authorities.

The invasion of Lebanon, Israeli officials have indicated, is linked to that effort. Without the PLO to encourage opposition and threaten those who cooperate, they say, Israeli occupation authorities could more easily find friendly Palestinians to work with.

This is particularly important in light of the Camp David talks among the United States, Israel and Egypt on Palestinian autonomy. So far, no West Bank or Gaza Palestinians have been willing to consider the type of autonomy under discussion in the long-interrupted negotiations. Moreover, if the talks fail, Israel has indicated it would consider imposing its own version of autonomy and would need cooperative Palestinians to work under it.

Since the invasion of Lebanon on June 6, mayors have been fired at Dura, near Hebron; Jenin, the West Bank's northern-most town, and now Gaza, on the Mediterranean near the border with Egypt. Even before then, the imposition of what the Defense Ministry calls a civil administration under a reserve military officer, Menachem Milson, had led to a crackdown beginning in March that included removing the mayors of Nablus, El Bireh, Ramallah and several smaller towns.

In May 1980, mayors Mohammed Milhem of Halhoul and Fahd Kawasme of Hebron were expelled .

In the past three weeks occupation authorities also have replaced the leader of the city council in Rafah, another Gaza Strip town, in a controversy generated in part by the return of the Sinai with new boundaries that split the town between Egypt and Israel.

"They are trying to prevent any expression of our national will and our interests," said Bassam Shakaa, the deposed mayor of Nablus. "But we will persist in carrying out the policies of our people."

The Israeli attack on the PLO guerrillas in Lebanon and today's removal of Shawa were both part of the same "aggression against our people's national establishments," Shakaa said.

Shakaa and mayor Karim Khalaf of Ramallah were left invalids by car bombs two years ago. Since their dismissals in March, both have attempted to remain active politically by speaking out. But deprived of an official platform and restricted by Israeli soldiers in their movements and meetings, their effectiveness as leaders has diminished sharply.

Both were elected in municipal elections in 1976 that swept a group of PLO supporters into office in major West Bank towns. Shawa, although appointed separately and given to moderate language, was considered part of that group.

Brig. Gen. Yousef Lunz, head of Gaza's civil administration, called in Shawa three days ago and told him to cease all contacts with the PLO and accused him of paralyzing Gaza's municipal government by ignoring the Israeli authorities. Shawa refused to abide by the warning.

Meanwhile, students at Bir Zeit University near Ramallah demonstrated several days this week. As a result, Israel closed the school yesterday for three months.

General strikes have been called several times by West Bank student groups and unions and demonstrations have broken out in some towns protesting the invasion of Lebanon. Two Palestinians were killed in such disturbances Sunday.