Heinz Fischer of the opposition Social Democrats promised to "observe all laws and the constitution of the republic" as he was sworn in as president in Austria's Parliament.

Fischer, 65, became Austria's first head of state from a leftist party in 31 years.

He won the largely ceremonial job in elections in April, defeating conservative Foreign Minister Benita Ferrero-Waldner of the ruling People's Party.

The ceremony in a parliamentary hall was restrained, however, as the nation mourned Fischer's predecessor, Thomas Klestil, who died Tuesday of multiple organ failure.

Lawmakers and most guests wore dark clothes and giant Austrian flag was adorned with a black ribbon.

Klestil, 71, died at a Vienna hospital where he was airlifted Monday after suffering heart failure. He was to be buried in a state funeral Saturday.

Several presidents, including Ireland's Mary McAleese, Hungary's Ferenc Madl and Poland's Aleksander Kwasniewski, planned to attend.

President Bush expressed condolences in a statement that said Klestil was known in the United States "as a man dedicated to freedom and human dignity, and as president he was a committed and eloquent advocate of these values."

Fischer bowed after being sworn in and was greeted with hearty applause and a few hesitant smiles but no jubilation. He began his first speech by thanking Klestil "for his life achievements."

The swearing-in ceremony was immediately followed by a special session of mourning.

Fischer's speech was interrupted by applause when he described how happy he was that his 95-year-old father-in-law, who survived a Nazi concentration camp, attended Thursday's ceremony.

The new president announced that he had suspended his membership in the Social Democrats so he could be a president representing all Austrians -- but added that he would not "lose sight of the necessary role of the opposition in a democracy."

Though the post is mostly ceremonial, the president is commander in chief of the country's military. The president's constitutional power to reject nominations for Cabinet ministers or even to remove them from office has rarely been used.

Fischer said that he, like many other Austrians who grew up during and after World War II, was very "sensitive to war and peace."

"Peace and politics to promote peace ... must have a central role in our efforts," he said.

Neutrality became an issue in the campaign leading to the April election, with Fischer's rival Ferrero-Waldner arguing it was time reconsider the policy, which was adopted by Austria in 1955. Fischer contended that remaining politically detached wouldn't prevent Austria from showing solidarity with other European countries in times of conflict.

Austria is one of four neutral EU members, along with Finland, Ireland and Sweden.

Fischer studied law and began his political career as a lawyer for the Social Democrats. In 1963, he became secretary of Parliament's Social Democratic delegates. He served as science and research minister from 1983 to 1987 and was elected president speaker in 1990. He became the deputy parliament speaker in 2002.

Fischer, an avid mountaineer who loves jazz, is married and has a son and a daughter.