Hamas leaders on Monday vowed to "cut off" the head of Prime Minister Ariel Sharon, after their spiritual leader Sheikh Ahmed Yassin was killed in a missile strike by Israeli gunships in Gaza City, reports Haaretz newspaper.

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The paper quoted leaders of the radical Islamic group as saying: "Sharon has opened the gates of hell and nothing will stop us from cutting off his head."

Hamas official Ismail Haniyeh, a close associate of Yassin in Gaza, was quoted as saying: "Words cannot describe the emotion of anger and hate inside our hearts."

Another Hamas leader said Islamic groups around the world will join forces for a retaliatory strike and indicated that the United States could be a target.

"The Zionists didn't carry out their operation without getting the consent of the terrorist American administration, and it must take responsibility for this crime," Hamas said in a statement.

Describing the scene in Gaza City, Haaretz correspondent Arnon Regular wrote: "Tens of thousands of Palestinians, many in tears, poured into the streets of Gaza City immediately after the strike. Gunmen fired into the air and militants threw dozens of pipe-bombs to express their outrage."

The Palestinian Authority has declared three days of mourning.