HADERA, Israel -- A Palestinian suicide bomber yesterday slipped into a crowd lining up for sandwiches in northern Israel, setting off a thunderous blast that killed five people, wounded more than 20 and dimmed hopes for diplomatic progress in the wake of Israel's withdrawal from the Gaza Strip.

The explosion, at a popular falafel stand near the entrance to an open-air market in the coastal town of Hadera, left pools of blood and scraps of flesh amid a scattering of melons, onions and apples from nearby produce stalls. A carpet of glass shards covered the ground.

The Palestinian militant organization Islamic Jihad asserted responsibility for the bombing, saying it was carried out to avenge the killing of one of the group's senior commanders by Israeli undercover troops two days earlier in the West Bank town of Tulkarm.

Israeli experts and officials expressed doubt, however, that a retaliatory attack could have been mounted so swiftly. Typically, suicide bombings involve a complex interplay of explosives procurers, recruiters, handlers and guides, requiring weeks of planning.

Palestinian officials swiftly condemned the bombing. Palestinian Authority President Mahmoud Abbas said in a statement that the attack "could widen the cycle of violence, chaos, extremism and bloodshed."

Israel said the attack illustrated the dangers faced by its citizens if tight restrictions on Palestinians' movement are eased. The government of Prime Minister Ariel Sharon also renewed demands that Mr. Abbas act decisively against Palestinian militant groups. "This murderous attack is urgent proof that the Palestinian Authority must once and for all dismantle and disarm the terrorist infrastructure," said David Baker, an official in the prime minister's office.

The attack, six weeks after Mr. Sharon's government completed the withdrawal of Jewish settlers and Israeli soldiers from Gaza, was the latest blow to hopes that the pullout would presage a return to the negotiating table by the two sides. The weeks since Israel ended its 38-year occupation of the narrow coastal strip have been characterized by slowly escalating violence, coupled with rancorous disputes between the two sides over control of Gaza's borders.

The Hadera attack also provides fresh ammunition for Mr. Sharon's political foes, who characterized the withdrawal as a capitulation to Palestinian militant groups that fought for years to drive Israel out of Gaza. The anti-pullout camp predicted that handing over Gaza would only embolden groups such as Hamas, the Al Aqsa Martyrs Brigades and Islamic Jihad.

Of the main militant groups, Islamic Jihad is the smallest and weakest, but in recent months has been particularly active in attacks against Israelis. The Hadera attack was the fourth suicide bombing in Israel this year -- and the fourth for which Islamic Jihad claimed responsibility.

The group has said it still considers itself a party to an informal "calm" declared seven months ago by the main militant factions, but Israel has made clear that it considers Islamic Jihad fighters to be fair game -- particularly if they are actively planning attacks.

Israel said that description fit Luai Saadi, a 26-year-old Islamic Jihad commander who was slain along with a top lieutenant in an Israeli raid on the Tulkarm refugee camp before dawn Monday. Israel said Saadi headed a cell responsible for bombings this year at a Tel Aviv nightclub and a shopping mall in the northern Israeli city of Netanya that killed a total of 10 people, and said Saadi had been helping to plan another attack.

Israeli commentators predicted the confrontation with Islamic Jihad would escalate.

The Hadera attack came the day after Simhat Torah, the last in a nearly monthlong string of Jewish religious holidays that make up the High Holy Days. Some Israelis began returning to work yesterday, but school was out and many people had the day off.