Palestinian security forces have arrested a Hamas activist who stashed nearly a ton of explosives in his home, right next to Palestinian police headquarters on the outskirts of Hebron, Palestinian officials said Thursday.

Hamas founder Sheik Ahmed Yassin acknowledged the Islamic militant group was having trouble making good on threats to carry out attacks against Israelis in retaliation for the killing of two Hamas fugitives by Israeli soldiers last month.Yassin said Palestinian and Israeli security forces had stepped up their campaign against Hamas. However, a Hamas "response will come at the appropriate time, even if it is delayed," Yassin told a news conference in Gaza City.

In the Israeli-controlled center of Hebron, several dozen Palestinian teenagers hurled stones and firebombs Thursday at Israeli soldiers who responded with rubber bullets and stun grenades. Two Palestinians were injured.

The army had imposed a curfew on the downtown area Wednesday after an assailant, apparently a Palestinian, threw two hand grenades at Israeli troops, injuring 11 soldiers and 11 Palestinian passers-by.

Israeli soldiers also sealed the Palestinian-run parts of town, barring Palestinians from entering or leaving. Hebron is home to 450 Jewish settlers and 130,000 Palestinians.

The assailant escaped into the Palestinian-controlled sector, apparently after being shot in the leg by an Israeli soldier. Israel filed a sharp protest with the Palestinian Authority, saying it must do more to catch the fugitive.

The attack boosted Prime Minister Benjamin Netanyahu's claims that Palestinian leader Yasser Arafat is not cracking down enough on militants.