Otto von Habsburg, the oldest son of Austria-Hungary's last emperor and the longtime head of one of Europe's most influential families, died Monday, his spokeswoman said. He was 98.

Habsburg died in his sleep at his home in Poecking in southern Germany, where he had lived in exile since the 1950s, spokeswoman Eva Demmerle told The Associated Press. A longtime advocate of European reunification, he campaigned against communism and for the removal of the Iron Curtain.

Born in 1912 in what is now Austria, Habsburg witnessed the family's decline after the empire was dismantled and Austria became a republic following World War I. He became head of the family at his father's death in 1922 and continued to claim the throne until the 1960s.

He was a member of the European Parliament for the conservative Bavarian Christian Social Union in southern Germany and also served as president of the Pan-European League from 1979 to 1999.

In that role, he was instrumental in helping organize the Pan-European Picnic peace demonstration in 1989 on the border of Austria and Hungary. The border was briefly opened in a symbolic gesture, which created the opportunity for 600 East Germans to flee communism months before the fall of the Berlin Wall.

It was the first time an Eastern European nation had opened its borders, and is widely seen as the start of the fall of communism.

European Commission President Jose Manuel Barroso mourned the passing of "a great European ... who gave an important impetus to the European project throughout his rich life".

"He made a central contribution to the opening of the Iron Curtain and the peaceful reunification of our continent that had been divided for too long," Barroso said in a statement. "I will particularly remember his strong stance against all forms of totalitarianism and on Europe's fundamental values."

Habsburg was born in the Austrian city of Reichenau, the oldest son of Charles I, whose family ruled much of central and eastern Europe for centuries.

But after Austria and Germany lost World War I, the Austria-Hungarian Empire was dismantled, Charles I had to resign and Austria went on to become a republic.

In 1919, Charles and his family had to leave the country for what turned out to be permanent exile in various European countries.

After his father's death in 1922, the 10-year-old Otto officially took over as the head of the House of Habsburg.

Otto tried to negotiate his return to Austria in 1935 and again in 1938 when he even sought to become chancellor to fight the expected invasion by Hitler's troops, but could not gather enough support.

Instead, he found a channel through the U.S. embassy in Paris to contact President Franklin D. Roosevelt and later claimed to have prevented Allied bombings of a number of Austrian cities by pleading with the U.S. military.

He was also credited with having helped about 15,000 Austrians escape the Nazis. At the same time, as he told the Austrian paper Die Presse in 2007, he negotiated Austria's postwar fate with Roosevelt, Winston Churchill and Charles de Gaulle.

Still, he was not allowed to return to Austria until 1966, five years after he officially abdicated the crown. He later claimed to be baffled by the hostility and criticism in his home country.

Habsburg's wife, Regina, died last year. The couple had seven children. Their eldest son, Karl, now runs the family's affairs and has been the official head of the House of Habsburg since 2007.

Habsburg will be buried July 16 in the Emperor Tomb in Vienna, below the Austrian capital's Capuchin Church.

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Gabriele Steinhauser in Brussels contributed to this report.