Leaders of Israel's peace movement said today they are forming a "self-defense militia" to counter terrorism by vigilante groups of ultranationalist Jewish settlers from the occupied West Bank, but they said they will attempt to avoid civil strife.

"We won't spill any blood, but if these [Jewish] terrorists continue to confront us, they will meet our fists," Meir Pail, a member of parliament from the left-wing Shelli Party, said at a news conference.

Pail said the militias, which he described as "protective guards" and "defense groups," will be unarmed and will serve primarily to protect Israeli peace groups that have been attacked or threatened by extremist settlers.

Pail and Uri Avneri, also a Shelli Party member of parliament, warned of a "reemergence of the armed and violent underground movement" that has its origins in Israel's prestatehood days. They said there is a "grave danger" of the violence spreading from the West Bank to Israel proper, and reaching even elected officials.

While avoiding the use of the term "civil war," Pail and Avneri said that the ultranationalist settlement movement in the West Bank has brought Israel to "the beginning stages of a nondemocratic struggle." Most of the settlers are Army reservists equipped with automatic weapons, and their vigilante groups are capable of threatening the government, they said.

Officials of Prime Minister Menachem Begin's government while voicing concern over recent vengeance attacks by Jewish settlers against West Bank Arabs, have sought to portray the phenomenon as controllable because, they say, it involves only small groups of extremists.

But the Shelli Party "self-defense group" represents the first time that Israeli peace activists have formally banded together to confront groups of Jewish settlers, and security officials said they feared it may provoke clashes.

Avneri said a "vast reservoir" of activists in Israeli's Peace Now movement was available to form small, unarmed units of three to 10 members each, ranging in age from 18 to 35. Almost all have military experience, he noted, but they will undergo training. They will also be prohibited from carrying weapons or initiating offensive actions, he said.

Instead, Avneri said, the groups will guard the headquarters of Shelli and other peace groups to prevent a repetition of recent attacks by rightist Jewish extremists.

On Sunday, Jewish vigilantes raided the Shelli offices in Tel Aviv, ransacking files and painting slogans on the walls. The previous day, Avneri said, extremist settlers attacked peace activists who had been seeking petition signatures on Tel Aviv's main thoroughfare, Dizengoff Street.

"We will confront them in the battlefield of the street if they continue their tactics. We will have guards to confront these new tendercies," Pail said. He said the militias will not cross into the West Bank to confront the settlers.

Both Pail and Avneri said they have received telephoned threats almost daily since the recent wave of violence in the West Bank, which peaked with May 2 ambush killing of six settlers in Hebron and the retaliatory bombing attacks on June 2 against three West Bank Arab mayors.

Pail blamed a new Jewish terror group, called Terror Against Terror, which is believed to be an offshoot of the revived Stern gang, a Jewish terrorist organization of the 1940s, for the threats and the attack on Shelli headquarters.

"Those who start terrorist activity in the West Bank will transfer it inside Israel against radical doves who struggle for peace," Pail said.From there, it is only a step away from attacking elected officials, including Begin, who is perceived by the ultranationalist settlers as having betrayed Zionism, Pail said.