World leaders expressed shock and sadness at the death of Poland's President Lech Kaczynski and many of the country's top state and military officials in a plane crash Saturday in western Russia.

Russia's Prime Minister Vladimir Putin and President Dmitry Medvedev were quick to offer condolences to Polish leaders and vowed a thorough probe into the crash of Kaczynski's plane in thick fog as it was landing at Smolensk airport.

Putin voiced sympathy over the "tragic" accident in a phone call with his Polish counterpart, Donald Tusk, said the Russian premier's spokesman Dmitry Peskov.

"Nothing like this has ever happened before," Putin said later at a meeting of the Russian cabinet devoted to the crash.

"I have given an order to carry out a detailed investigation of the accident with complete and close cooperation with the Polish side," Medvedev said in a message to Bronislaw Komorowski, speaker of the Polish parliament who is now acting head of state.

In an address to be broadcast on Polish television, Medvedev said: "In the name of the Russian people, I offer my deepest, truest condolences to the Polish people, feelings of compassion and support for the relatives and loved ones of those killed."

He said Monday would be declared a national day of mourning in Russia.

A total of 97 people were on board the plane, including the heads of Poland's armed forces, the central bank governor and members of parliament, flying in to attend a memorial ceremony for the massacre of 22,000 Polish army officers by Soviet secret police during World War II.

Many were buried in the Katyn forest, near Smolensk.

Poland's first post-communist president Lech Walesa cried, "Jesus, Holy Mary, it's an inconceivable tragedy, an inconceivable disaster," when he learned the news.

"The Soviets killed Polish elites in Katyn 70 years ago. Today, the Polish elite died there while getting ready to pay homage to the Poles killed there," Walesa told AFP.

UN Secretary General Ban Ki-moon was "shocked to hear the news of the plane crash in which Polish President Lech Kaczynski, his wife and many other people, including senior officials, were killed," his office said, extending condolences to Poland and especially the families of those who perished.

US President Barack Obama described the loss as "devastating to Poland, to the United States, and to the world."

He praised those killed in the crash as "many of Poland's most distinguished civilian and military leaders who have helped to shape Poland's inspiring democratic transformation."

Also offering condolences was former president George W. Bush, who called Kaczynski "a strong defender of freedom and a friend to the United States," he said.

German Chancellor Angela Merkel said she was "deeply shocked", adding, "It is a human and political tragedy for our neighbour Poland."

French President Nicholas Sarkozy said he was deeply moved and very sad at the death of Kaczynski, a man "driven by ardent patriotism, who dedicated his life to his country."

Britain's Queen Elizabeth II sent her "deepest sympathy" to the Polish people and Prime Minister Gordon Brown said Kaczynski would be mourned across the world and remembered as "a passionate patriot and democrat."

Eurosceptic Czech President Vaclav Klaus, the conservative Kaczynski's closest ally among European politicians, said he had lost "a real friend," calling the "death "an absolutely terrifying event that has shaken me, shocked me, made me sad and touched me personally," Klaus told reporters.

Pope Benedict XVI also said he felt "deep pain" at the death of the head of state of strongly Catholic Poland, expressing condolences and calling for special blessings on the Polish people.

European Commission President Jose Manuel Barroso called Kaczynski "a very determined Polish patriot", and the European Union has declared a day of mourning in Brussels on Monday, said a government source in Spain, which holds the rotating EU presidency.

Others who expressed their shock and sympathy included leaders of Bulgaria, Israel, Italy, Lithuania, the Netherlands, Portugal, Slovakia, Spain, Sweden, Romania, Switzerland, Ukraine, Balkan countries, Turkey, Georgia and Iran.

From Asia, China's President Hu Jintao said he was "deeply saddened" while President Hamid Karzai of Afghanistan said his heart went out to the Polish people and the families of those killed.

In Islamabad President Asif Ali Zardari described Kaczynski as "a respected leader and good friend of Pakistan" and said his country shared "the grief and loss of the people of Poland."

Similar messages came from the North Atlantic Treaty Organisation, the International Monetary Fund, the Organisation for Security and Cooperation in Europe and the Council of Europe.