Israel rounded up more than 30 Hamas leaders in the West Bank on Thursday, including a Cabinet minister, taking its conflict with Hamas over daily Palestinian rocket fire from Gaza to a new level, drawing condemnation from the moderate Palestinian president and threats of punishing Hamas attacks inside Israel.

The arrest of 33 Hamas leaders reflected an Israeli decision to target the Islamist group's political leadership — but not necessarily in lethal airstrikes.

On Thursday, Israel carried out several airstrikes in Gaza, all directed at Hamas training bases and command posts. A huge plume of black smoke rose over Gaza City after a mid-afternoon strike, but there were no serious injuries, Palestinian medics said.

At sundown Thursday, a mortar shell fired from Gaza exploded at Erez, the main crossing for people, and Israel closed the crossing, the military said. There was considerable damage but no one was hurt.

More than 40 Palestinians have been killed in Israeli raids over the past 10 days, and an Israeli woman was killed by a rocket on Monday. The rocket barrages have severely disrupted life in the area near Gaza. Thousands of frightened residents have fled.

Israeli Defense Minister Amir Peretz said the arrests were part of Israel's attempt to neutralize Hamas.

"Arrests are better than shooting, " he told Israeli Army Radio. "The arrest of these Hamas leaders sends a message to the military organizations that we demand that this firing (of rockets) stop."

But Hamas remained defiant. "We will chase the occupation soldiers and the settlers in every inch of our occupied land, and we announce that we give free hand to our cells to strike against the enemy in every place in Palestine," a Hamas term that includes Israel, the Islamist group said in a statement.

Palestinian President Mahmoud Abbas said the arrests were a blow to peace efforts, and a spokesman for Palestinian Prime Minister Ismail Haniyeh, of Hamas, demanded immediate release of the detainees and called on the U.N. and European Union to impose sanctions on Israel.

Visiting EU foreign policy chief Javier Solana met Abbas in Gaza Thursday and called for both sides to halt the violence.

"The rockets and the Israeli response have to stop," he told reporters after the meeting.

Abbas himself condemned what he called the "absurd" rocket fire and said he was trying to persuade militant groups to stop. "They must stop so we can reach a truce with Israel," he said, adding that the Israeli airstrikes were failing to stop the salvos.

Just after the meeting, Israeli forces carried out two more airstrikes, hitting an empty Hamas base in Gaza City and a base in central Gaza, Palestinian security officials said. Four civilians were lightly injured by flying glass and debris in Gaza City, medics said. No one was hurt in the second strike. The Israeli military said it hit Hamas emplacements.

The most prominent leader arrested overnight was Education Minister Nasser Shaer, considered a pragmatist. His wife, Huda, said soldiers knocked on the door of their home in the West Bank city of Nablus and took him away. Troops also seized Shaer's computer, she said. Israel also detained Shaer for a month last year, before a judge ordered his release.

Israel has been holding 40 Palestinian lawmakers from Hamas, including the parliament speaker, Abdel Aziz Duaik, rounded up over the past year. Previous arrests of notables were linked to the capture of an Israeli soldier by Hamas-linked gunmen in a cross-border raid that killed two other soldiers in June 2006. The idea was to trade the lawmakers for the soldier, but no deal took place.

Also among those rounded up Thursday were former Cabinet minister Abdel Rahman Zeidan, legislators Hamed Bitawi and Daoud Abu Ser, the mayors of the towns of Nablus, Qalqiliya and Beita, and the head of the main Islamic charity in Nablus, Fayad al-Arba.

So far, Israel's other tactics against the rocket fire have failed. In the past, even large-scale ground invasions to take over areas where militants launch rockets, while causing multiple casualties and widespread damage, have not stopped the barrages.

Also, Abbas' security forces have been ineffective in halting the rocket salvos and have made few visible efforts in that direction, despite Abbas' frequent denunciations.

Instead, the rocket attacks have diminished when Palestinian militants decide to lower the flame. Intentionally or not, the sudden flareup with Israel has stopped a month of bloody internal clashes between Hamas and Abbas' Fatah forces.

The military said Hamas fired six rockets at Israel on Thursday, though Hamas claimed firing 14, still less than previous days. Channel 10 TV's military analyst, Alon Ben-David said the dropping number was a hint that Hamas may have achieved its goals and is ready to wind down the crisis. Ben-David added, however, that if Israel kills a top Hamas commander in an airstrike, that would set it off again.