In a public statement released on Sunday, European Union foreign affairs chief Javier Solana said he was at a complete loss as to the benefit of the assassination and was unable to see any positive outcome.
"Actions of this type are not only unlawful, they cannot be conducive to lowering tension."
UN Secretary-General Kofi Annan said that "extrajudicial killings are violations of international law" and called on Tel Aviv to immediately end the illegal practice.
British Foreign Secretary Jack Straw said such assassinations were wrong and unhelpful to peace.
"The British government has made it repeatedly clear that so-called targeted assassinations of this kind are unlawful, unjustified and counterproductive," he said.
Italian Foreign Minister Franco Frattini said his country "like the whole of the European Union, has always condemned the practice of targeted assassinations, which contribute to furthering the spiral of hatred and violence".
Ruined peace plan?
The EU was particularly angry at the Israeli action coming just hours after its officials had announced a high-level meeting of the diplomatic quartet promoting a Middle East peace plan.
The 4 May meeting was hoping to deal with the latest developments after the United States, in a dramatic policy reversal, backed unilateral measures planned by the Israeli government.
Arab League chief Amr Mussa said: "The continuation of this Israeli policy of aggression demands serious action from the international community to secure international protection for the Palestinian people."
A US-made Apache helicopter
left little of al-Rantisi's car
Iranian Vice President Muhammad Ali Abtahi, calling al-Rantisi a "martyr", said the assassination was a "terrible crime" that "will only give fresh blood to the resistance in the region".
Jordanian government spokeswoman Asma Khudr denounced the killing as an "odious crime" and considered it "a loss in hopes for achieving peace in the region".
The United States claimed it "certainly did not give Israel any kind of green light" to assassinate Hamas leader al-Rantisi.
A US State Department official told CNN television on Sunday Washington did not have advance knowledge of the Apache helicopter rocket attack that killed the spokesman.
"All along, the United States has urged Israel to consider the consequences of its actions. The US did not think the assassination of Shaikh Yasin was well considered."
But scepticism at US ignorance of assassination plans was obvious in the Arab world, and no public figure in the EU, the UN or the Middle East has condoned the attack.
The assassination came on the heels of President George Bush's endorsement on Wednesday of Israeli plans to keep illegal settlements and the denial of a Palestinian right to return.