UPDATE: Poland struggles with shock of leaders' deaths

SMOLENSK, Russia (AFP) – A plane carrying Polish President Lech Kaczynski and much of the nation's ruling elite crashed landed in the Russian city of Smolensk on Saturday killing all 96 people on board.

Investigators scrambled on Sunday to determine if pilot error was to blame for the fiery crash.

The disaster sent Poland into shock.

Mourners in Warsaw turned out by the tens of thousands on Saturday to sing the national anthem, wave the red and white Polish flag, and lay a carpet of candles and flowers at the presidential palace and nearby Pilsudski Square.

"It's my heart that told me to come here, I simply had to come," said 45-year-old translator Anna Ciostek, who came to the square with her daughter and mother. Scene: Tears on the streets for Poland's tragic president

Polish Prime Minister Donald Tusk and the late president's twin brother, former prime minister Jaroslaw Kaczynski, visited the crash site late on Saturday evening, accompanied by Russian Prime Minister Vladimir Putin.

The remains of all the victims were recovered and transferred to Moscow in two heavy-lift military helicopters for identification, as next of kin began to arrive in the Russian capital. Scene: Plane lies mangled in 'forest of death'

"Everything must be done to establish the reasons for this tragedy in the shortest possible time," said Putin when he arrived in Smolensk, about 75 kilometres (45 miles) from the Belarus border.

Besides the 60-year-old head of state, the mid-morning crash killed his wife Maria, as well as Poland's military chief of staff, other senior officers, its central bank govenor, deputy foreign minister and members of parliament. Facts: Polish leaders killed in crash

"This kind of dramatic tragedy is unheard of in the modern world," Tusk said before heading to the crash site.

Thrust into the forefront as acting president was Bronislaw Komorowski, speaker of the Polish parliament.

"It is rare for so many high ranking representatives of the highest military and spiritual leaders, and the president himself, all to die at the same moment," he told AFP.

"We have to deal with this very difficult problem -- and we will."

The Tupolev Tu-154 jet was carrying Kaczynski and his wife, the military chief of staff, central bank governor, deputy foreign minister, members of parliament and other top officials to a ceremony for thousands of Polish troops massacred by Russian forces in World War II.

The Russia-built jet hit treetops in fog as it approached Smolensk, crashed and broke up in flames, Smolensk regional governor Sergei Antufiev said.

"It clipped the tops of the trees, crashed down and broke into pieces," Antufiev told Russia-24 television news network. "There were no survivors."

Russian news agencies said pilot error had come under immediate suspicion.

The wreckage was scattered across a forest with parts of it still on fire, Russian television images showed. Firefighters doused the flames.

Former Polish president Lech Walesa, who headed the Solidarity movement, called the crash "inconceivable".

"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," a shaken Walesa told AFP.

The passengers included General Franciszek Gagor, chief of Poland's armed forces, central bank governor Slawomir Skrzypek, deputy foreign minister Andrzej Kremer, Kaczynski's wife Maria, and scores of MPs, historians and other officials, Polish officials said.

All were to attend a memorial service in the Katyn Forest, near the crash scene, where 22,000 top Polish officers and troops were killed by Soviet troops 70 years ago. It had been intended to help the reconciliation between Poland and Russia.

The jet's engines and a large chunk of its mud-caked tailfin, strewn over a large area in the fog-blanketed forest.

Firefighters hosed down burning sections of the plane while security personnel and civilian investigators inspected the wreckage.

Officials in Warsaw confirmed that Kaczynski was aboard the plane and Russian television broadcast video shot earlier Saturday of the president and his wife boarding the plane in Warsaw.

Russian news agencies reported that pilot error was suspected. "The cause of the plane crash was apparently an error by the crew during the approach to landing," Russian state news agency RIA Novosti quoted an unnamed official in Smolensk as saying.

Russian President Dmitry Medvedev immediately appointed Prime Minister Vladimir Putin to head a commission of investigation and sent Russia's emergency situations minister, Sergei Shoigu, to the site.

The aircraft crashed a few hundred metres (yards) short of the runway at the Severny airport outside Smolensk, ITAR-TASS news agency reported, quoting rescuers at the site.

The flight data recorders had not yet been located but experts were on the scene and the search for them was under way, ITAR-TASS said.

Kaczynski, the identical twin brother of former prime minister Jaroslaw, was on his way to attend commemorative ceremonies for the Katyn massacre, which decimated Poland's military and intellectual elite 70 years ago.

The crash occurred three days after Putin and his Polish counterpart, Donald Tusk, together attended a memorial for the victims of the massacre at Katyn.

The Putin-Tusk meeting there was seen as a huge symbolic advance in Russia's often thorny relations with Poland.

World leaders immediately expressed shock at the disaster.

French President Nicholas Sarkozy said he "conveys all his sympathy to the family of president Kaczynski and to the families of all the victims and wishes to express his deep personal condolences and on behalf of the French people."

He paid homage to his Polish counterpart as a man "driven by ardent patriotism, who dedicated his life to his country."

German Chancellor Angela Merkel said: "I am deeply shocked by the plane accident and the death of the Polish president."

"I think everyone around the world will be sending their sympathies to the people of Poland," said British Prime Minister Gordon Brown.