"I listened to the conversation, which I think is without precedent, when our premier talked at the same time with the premier and the president of Russia, who handed the receiver back and forth to each other," Mr Sikroski said.

President Medevedev in an emotional TV statement said that: "In the name of the Russian people I send the Polish people my deep and sincere sympathy and promise to help the bereaved and relatives of the victims."

From the US, President Obama said in a written statement: "It is a testament to the strength of the Polish people that those who were lost were travelling to commemorate a devastating massacre of World War II as the leaders of a strong, vibrant and free Poland.

"That strength will ensure that Poland emerges from the depths of this unthinkable tragedy, and that the legacy of the leaders who died today will be a light that continues to guide Poland - and the world - in the direction of human progress."

German leader Angela Merkel said: "This is a political and human tragedy for Poland our neighbour ...We in Germany will miss him [Mr Kaczynski] too."

Similar tributes came in from the UK's Gordon Brown, France's Nicolas Sarkozy, their Czech and Lithuanian counterparts and the Nato secretary general.

Speaking on behalf of the EU, Mr Barroso said: "I have worked very closely with President Kaczynski, in a spirit of loyalty, and I respected in him a very determined Polish patriot who at the same time was very committed to our European Union and to the values of freedom and solidarity."

EU foreign affairs chief Catherine Ashton has postponed a scheduled trip to Warsaw on 12 April. All meetings of EU diplomats in Brussels on Monday will start with a moment of silence and flags on the EU Council building will carry black ribbons.

Pope Benedict XVI offered a special blessing for the Polish nation. Israeli premier Benjamin Netantyahu praised Mr Kaczynski for opening a "new chapter" in Polish-Jewish relations.

The cause of the crash remains unknown, but eyewitness accounts indicate it was due to pilot error as the plane tried to land in fog at an airstrip in a forest near Smolensk in western Russia.

Mr Kaczynski was en route to solemnities for the 70th anniversary of the Katyn massacre, in which Soviet secret police murdered 22,000 Polish officers and intellectuals. His trip was to mark a rapprochement in Polish-Russian relations.

"I think that this [Saturday's] catastrophe will again influence Polish history," former Czech president Vaclav Havel said.