Israel is bracing for possible retaliatory attacks following Monday's targeted killing of Hamas spiritual leader Sheikh Ahmed Yassin in an Israeli helicopter missile attack.

Israeli forces are on high alert. Troops and tanks also moved into the northern Gaza Strip late Monday. Officials said the action was to taken to stop Palestinian rocket attacks.

Palestinian witnesses say Israeli tanks took up positions near the town of Beit Hanoun - an area where Palestinians fired a number of homemade rockets into Israel. The rockets caused no damage or casualties.

Meanwhile, Palestinians have begun three days of mourning for Sheikh Yassin, who was killed as he was leaving a Gaza City mosque after dawn prayers Monday. Palestinians say seven other people, including two of Sheikh Yassin's bodyguards, were also killed in the missile attack.

Tens of thousands of Palestinians joined in a massive funeral procession in Gaza City. And protest marches were held throughout the Gaza Strip and West Bank.

Hamas and other militant groups, including the Al Aqsa Martyrs' Brigades, have vowed revenge.

Israel said Sheikh Yassin was personally responsible for many suicide attacks against Israelis.

The Palestinian Authority condemned what it called Israel's "dangerous" and "cowardly" act.

The United States said it is "deeply troubled" by the killing but stopped short of outright condemnation.

Palestinian and Arab representatives to the United Nations are asking the Security Council to condemn the Israeli action.

U.N. Secretary-General Kofi Annan, the European Union's foreign policy chief Javier Solana and many countries around the world said the killing will not help lead to Middle East peace.