UPDATE 2-Poland to bury president in Krakow, Medvedev arrives

* Kaczynski and wife to be buried alongside kings, heroes

* Ash cloud forces Obama, Merkel to scrap plans to attend

* Russia's President Medvedev heads to Krakow

(Recasts, adds Medvedev arrival, details)

By Gabriela Baczynska and Wojciech Zurawski

KRAKOW, Poland, April 18 (Reuters) - Tens of thousands of mourners gathered in Poland's ancient capital on Sunday for the burial of President Lech Kaczynski and his wife, but volcanic ash over Europe prevented many world leaders from joining them.

U.S. President Barack Obama and German Chancellor Angela Merkel were among those forced by the ash cloud to abandon plans to attend the funeral in Krakow for Kaczynski and his wife Maria, killed in a plane crash in western Russia on April 10.

However, Russia's President Dmitry Medvedev managed to fly to the city, reinforcing a strong message of Russian solidarity since the crash that has raised Polish hopes for an improvement in long-strained ties with their communist-era overlord.

The funeral at Krakow's Wawel cathedral crowns a week of unprecedented national mourning for the Kaczynskis and 94 other, mostly senior political and military officials who also died.

Police said about 60,000 people had gathered in central Krakow ahead of the transportation of the Kaczynskis' coffins to Wawel cathedral at 1330 GMT where they would be placed in the crypt alongside Polish kings, national heroes and poets.

In Warsaw, Poles had queued through Saturday night to view the coffins while they remained on public display.

The coffins were taken to Krakow by military plane early on Sunday, flying at low altitude because of the ash cloud that has shut the airspace of Poland and many other European countries to commercial traffic.

Kaczynski's twin brother Jaroslaw, a former prime minister, and other family members insisted the funeral go ahead on Sunday as planned. The family, Prime Minister Donald Tusk and interim President Bronislaw Komorowski travelled to Krakow by train.

REGRETS, SOLIDARITY

Obama said he regretted being unable to attend the funeral. U.S. Ambassador Lee Feinstein was due to represent him.

"President Kaczynski was a patriot and close friend and ally of the United States, as were those who died alongside him, and the American people will never forget the lives they led," Obama said in a statement shortly before he had been due to fly.

Poland, part of the Soviet bloc during the Cold War, is now a member of NATO and a close U.S. ally.

Merkel of Germany, Poland's western neighbour and biggest trade partner, also expressed regret at having to call off her trip. Germany's president and foreign minister flew to Krakow from Berlin by helicopter for the funeral.

Leaders of other relatively nearby countries such as Ukraine and the Czech Republic also were expected to attend.

Medvedev's presence was ironic, given that Kaczynski was a stern critic of what he called Russia's "imperialism" towards ex- Soviet republics such as Georgia and Ukraine.

Kaczynski flew to Georgia during its short 2008 war with Russia to demonstrate Polish solidarity with President Mikheil Saakashvili. The Georgian leader was due to attend his funeral.

When their plane crashed in thick fog, Kaczynski and his entourage had been heading to Katyn forest in western Russia to commemorate the 70th anniversary of the massacre of 22,000 Polish officers and intellectuals by the Soviet secret police.

Wawel cathedral was the coronation site of virtually all of Poland's monarchs and the adjacent castle was the centre of government for five centuries until the end of the 16th century.

Some Poles have staged protest rallies and joined petitions on social media site Facebook against the decision to bury Kaczynski in such a hallowed spot.

Kaczynski, president since 2005, was a polarising figure whose support levels had fallen to about 20 percent before his death. He had been expected to lose a presidential election due in the autumn and now likely to be held on June 20.

The protests were the first cracks in an otherwise remarkable display of national unity since the crash.

(Writing by Gareth Jones; editing by Michael Roddy)