World fears after Yassin killing
The killing of Sheikh Yassin has drawn widespread international condemnation.
United Nations Secretary General Kofi Annan said it was not only against international law, but did nothing to help find a peaceful solution.
Egyptian President Hosni Mubarak described it as cowardly and King Abdullah of Jordan called it a crime.
In Brussels, European Union foreign ministers condemned Israel for an "extra-judicial killing", which they said undermined the rule of law.
But the US State Department avoided direct criticism of Israel, urging all parties in the conflict to remain calm and show restraint.
White House spokesman Scott McLellan later called on all sides to "remain focused on measures that bring about an end to violence and terror and help to resume progess towards peace".
He added: "The Palestinian Authority needs to do everything it can to confront and dismantle terrorist organisations - that's the foundation of moving forward on the two-state vision."
'Spiral of violence'
British Foreign Secretary Jack Straw described the assassination of Sheikh Yassin as "unacceptable" and "unjustified".
Mr Straw said he did not think Israel would benefit from an attack on an old man in a wheelchair.
French Foreign Minister Dominique de Villepin said such acts could only "feed the spiral of violence".
The European Union's foreign policy chief, Javier Solana, said the killing was "very, very bad news" for the Middle East peace process.
In Moscow, the Russian foreign ministry said it was "deeply concerned" at the assassination, which it feared would cause "a new wave of violence".
Egyptian President Hosni Mubarak said he had cancelled a visit by Egyptian members of parliament to Jerusalem in protest at the assassination.
The delegation was to have taken part in celebrations to mark the 25th anniversary of the Camp David peace treaty between Egypt and Israel.
Mr Mubarak described the killing as "regrettable and cowardly".
When asked about its possible impact on the Middle East peace process, he said: "What peace process?"
Roadmap fears
Other leading figures in the Arab world have also criticised the killing of Sheikh Yassin.
Arab League Secretary General Amr Moussa said it was "state terrorism in its most hideous form", while Iraq's US-appointed Governing Council said it could escalate danger in Iraq.
One member of the Council, Muwaffaq al-Rubaiye, told the AFP news agency: "We condemn the killing, which will only serve to strengthen the justifications for terrorist acts in the world and does not serve peace."
Kuwaiti Prime Minister Sheikh Sabah al-Ahmad al-Sabah said: "Violence will increase now because violence always breeds violence."
Lebanese President Emile Lahoud said Israel was "mistaken" if it thought that by killing resistance fighters, it could kill the Palestinian cause.
Iran's ex-President Akbar Hashemi Rafsanjani warned that Sheikh Yassin's "martyrdom" would trigger an "even more serious struggle by the oppressed Palestinians against the Israeli terrorist occupiers and their US supporter".
The spiritual leader of Egypt's outlawed Muslim Brotherhood, Mohammed Mehdi Akef, said Sheikh Yassin had fallen as a "martyr" in a "cowardly operation".
BBC diplomatic correspondent Barnaby Mason says the world's big powers are likely to be dismayed that Israeli Prime Minister Ariel Sharon has chosen to escalate Israeli military action against Hamas.
Our correspondent says there will also be concern, especially in the British government, that the killing might further hinder the progress of the international roadmap peace plan for the Middle East.