The killing of Sheikh Yassin has drawn widespread international condemnation.
Sheikh Yassin's killing has been widely condemned
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.
The Bush administration said it was deeply troubled by the assassination, though it stopped well short of condemning it.
State Department spokesman Richard Boucher said it would not help the push towards peace in the region, while the White House appealed for restraint on all sides.
The BBC's Rob Watson says the killing seems to have put the US in an awkward spot as it tries to reconcile its fierce opposition to Hamas, which it regards as a terrorist organisation, with its disapproval of Israel's policy of targeted killings.
'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?"
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".