Israeli security forces said today they arrested 70 Palestinian terrorist suspects in the Gaza Strip, including the son of a well-known Palestine Liberation Organization supporter whom Israeli Foreign Minister Moshe Dayan met last month for an "exchange of views."

It was the most extensive crackdown in more than a year in Gaza, which once was regarded as a breeding ground of Palestinian guerrilla activity but which has been relatively quiet for more than five years.

Israeli army officials said a terrorist cell of Fatah, the largest faction in the PLO, was broken up in Gaza city and that seven explosive devices were uncovered. Authorities said members of the group admitted responsibility for the bombing of a bank on Aug. 26, and that under questioning they disclosed plans for two more attacks.

Officials said another Fatah cell was raided in Khan Yunis in the southern part of the strip, and that security forces also detained members of George Habash's Popular Front for the Liberation of Palestine.

Among those arrested last night was Tariq Shafi, son of Haider Abdul Shafi, a Gaza physician who before the 1967 Arab-Israeli war was chairman of the PLO parliament in Gaza and who after the Israeli occupation was expelled from Gaza twice for illegal political activity.

Dayan's visit to Gaza to meet with Shafi, coupled with a similar meeting in Hebron with a leading West Bank communist, Ahmed Hazmi Natshe, was widely criticized here as being imprudent. Natshe also had been deported for alleged incitement against the military authorities and he is a member of the Palestine National Council.

Dayan's critics raised comparisons between the foreign minister's talks with Shafi and the meeting in New York between U.S. Ambassador to the United Nations Andrew Young and the PLO's observer at the United Nations, Zehdi Labid Terzi, which Israel had condemned in an official protest.

Overlooked in the furor, which the Israeli press has covered extensively, is Dayan's longstanding practice of initiating private talks with Palestinian leaders, including PLO supporters, ever since Israeli occupation of the West Bank and Gaza 12 years ago.

A source close to the foreign minister said yesterday Dayan has held at least eight meetings with Palestinian leaders in the West Bank and Gaza since the Egyptian-Israeli peace treaty was signed on March 25 and that there had been similar meetings shortly before the Camp David summit conference last September. Those meetings were never publicly disclosed, said the source, who added that Dayan would most likely hold further meetings with Palestinians.

The purpose of the meetings, the aide said, was to prepare for the autonomy negotiations by sounding out Palestinian attitudes on alternative autonomy proposals at a time when negotiations on self-rule seem bogged down.

At present, not one significant Palestinian leader in the West Bank or Gaza has indicated a willingness to participate in Prime Minister Menachem Begin's limited autonomy plan, and Israeli officials have already begun talking privately about the need to devise contingency approaches in the event of failure.

"If he [Dayan] or anybody else is going to come up with any fresh ideas, it makes sense to talk with the people involved," a source close to Dayan said.

Asked whether Dayan's motive might be to establish a record of actively soliciting Palestinian participation in the autonomy talks in anticipation of the negotiations collapsing, the adviser said, "If you mean his getting an alibi, that is something that just happens to go along with it. It is not the purpose of these discussions."

Another motive that has been ascribed to Dayan, and which Foreign Ministry officials deny, is that Dayan is attempting to sow dissension among political figures in the occupied territories, or that he is attempting to signal Jordan's King Hussein that while Hussein stubbornly continues to boycott the peace process, Israel may overcome the impasse by approaching the West Bank leaders directly and even by unilaterally withdrawing its military government from the West Bank and Gaza.

Israeli officials in the past have repeatedly alluded to the possibility of an arbitrary pullout of the occupation government -- but not its security forces -- as a method of forcing the Palestinians to accept limited autonomy.

Shafi has said that Dayan raised that possibility in their talks, indicating that Dayan appeared to be seeking his reaction to such a move.

In a related development, Israeli Energy Minister Yitzhak Modai tonight said on Israel television that he would prefer that the government negotiate with Palestinians, including PLO supporters, rather than with Jordan, which at this stage, Modai said, would insist on extracting a "pound of flesh" and impede the peace talks.

Modai said it would be "political lunancy" for Isreal not to seek negotiations with Palestinians, including PLO sympathizers.