However, the biggest crackdown on militants since Palestinian leader Mahmoud Abbas took office a year ago has netted only low-level operatives, and some suspect the goal is to appease the United States and Israel rather than crush the militant group.

At the same time, analysts and Israeli security officials said the arrests have sent an important message to the Palestinians and Israelis that militant groups can no longer operate with impunity.

"It is a symbolic way to tell everybody, 'I am serious,'" said Israeli security analyst Boaz Ganor.

The last major Palestinian crackdown on militant groups took place four years ago when former leader Yasser Arafat, under intense pressure from Israel to stop attacks, ordered his security forces to arrest more than 200 Hamas and Islamic Jihad militants, placed Hamas founder Sheik Ahmed Yassin under house arrest and closed down many Hamas offices in the West Bank and Gaza Strip.

Since Abbas took office in January following Arafat's death, he has resisted calls by Israel and the United States to dismantle the armed groups, fearing it would touch off a civil war. Others are concerned that his myriad and competing security forces, devastated after five years of fighting with Israel, would be unable or unwilling to take on the militants.

Instead, Abbas negotiated a shaky cease-fire with the militants to end attacks on Israel. It greatly reduced violence but has been repeatedly violated by Islamic Jihad.

After Islamic Jihad carried out its fifth suicide bombing since the truce took effect in February, killing five Israelis outside a mall in the Israeli city of Netanya last week, Abbas suddenly promised to take action.

"The Palestinian Authority will not go easy on whoever is proved to be responsible for this operation," Abbas said in a statement.

In the following days, Palestinian security forces, many of them masked to protect themselves from reprisals, fanned out across the West Bank, surprising Islamic Jihad members at their houses in the middle of the night and arresting about 100 people.

Islamic Jihad has demanded an end to the raids. "These fighters were arrested to appease Israel," said an Islamic Jihad leader known only as Abu Majd, who implied that more arrests could spark retaliation from the militants.

Islamic Jihad leaders said many of those arrested were students, academics or political leaders of the group. Israel, which has been conducting its own simultaneous crackdown, has been targeting hard-core militants, they said.