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This year's Nobel Peace Prize has been awarded to the Chinese human rights activist Liu Xiaobo. Mr Liu will not be there to pick up the award at the ceremony on Friday in Oslo, as he is serving an 11-year prison sentence for subversion.

Previously, four other Nobel Peace Prize winners have been unable to pick up their awards in person.

Aung San Suu Kyi (Burma) - winner of Nobel Peace Prize in 1991

Burma's Aung San Suu Kyi was awarded the Nobel Peace Prize in 1991, a year after her National League for Democracy won an overwhelming victory in elections - a vote that was nullified by the junta.

At the time of her award, she was under house arrest and heard the news on the ���˿���, which was "a little strange," she says.

"It had something to do with me obviously, and yet it seemed to have nothing to do with me because it was so distant," she told ���˿��� World Service.

She says the award "heartened" her supporters at a difficult time. Her son Alexander Aris attended the ceremony in Oslo in her place, and made a speech on her behalf.

Aung San Suu Kyi was released last month after spending most of the last 20 years in detention.

"I would now dearly love to go to Oslo to thank the people of Norway for their strong support," she says.

Aung San Suu Kyi told the ���˿��� she would like to "hold out a hand of sympathy" to Liu Xiaobo, and she said it was "very sad" that his chair would be empty at Friday's award ceremony.

Lech Walesa (Poland) - winner of the Nobel Peace Prize in 1983

The Polish trade union leader and later president, Lech Walesa, was awarded the Nobel Peace Prize in 1983 for his peaceful opposition to Communism.

He was one of the founding members of Solidarity, and spearheaded the East European anti-Communist movement.

Lech Walesa decided not to attend the ceremony in Oslo for fear that he would not be allowed back into Poland. His wife and eldest son went in his place.

In 1981, Mr Walesa had been arrested and later released as part of an anti-democracy crackdown.

His son Jaroslaw Walesa told ���˿��� World Service that his father's award was a "tremendous help" to Poland and came at a time when "belief in peaceful change was just dissolving".

In 1984 Solidarity was legalised, and in 1990 Lech Walesa was elected president of Poland, a post he held for five years.

Mr Walesa offered to collect the award for this year's Nobel Peace Prize winner Liu Xiaobo in his absence.

Andrei Sakharov (Soviet Union) - winner of Nobel Peace Prize 1975

In 1975, nuclear physicist and human rights activist Andrei Sakharov became the first Soviet citizen to be awarded the Nobel Peace Prize.

His work in the 1950s helped lead to the creation of the first hydrogen bomb. Then in the 1960s and 1970s he became politically active, calling for drastic reductions in nuclear arms and co-founding the Moscow Human Rights Committee.

Soviet authorities did not allow Mr Sakharov to travel to Oslo to collect the award.

His wife Yelena Bonner, who went to Oslo in his place, described it as, "a great emotional burden; a pleasing and important task from my husband".

She told ���˿��� World Service that the award helped focus international attention on the situation in the Soviet Union, and especially those referred to at the time as "dissidents".

Andrei Sakharov died in 1989. The Sakharov Prize for Freedom of Thought is awarded annually by the European Parliament to individuals working on human rights.

Carl von Ossietzky (Germany) - winner of Nobel Peace Prize 1935

The German journalist and pacifist Carl von Ossietzky was awarded the Nobel Peace Prize in 1935, but he was prevented from travelling to the ceremony in Oslo by the authorities in Nazi Germany, who regarded him as a traitor.

He was a highly controversial choice at the time. It was the first time the winner had not been able to attend in person, and the first time the King of Norway had stayed away.

In 1931, Carl von Ossietzky was sentenced to 18 months in prison for an article he had published in his magazine. He was released, but soon afterwards was sent to the Sonnenburg concentration camp.

His friends lobbied hard for him to be considered a candidate for the Nobel Peace Prize, in the hope that this would secure his freedom.

Carl von Ossietzky was transferred to a hospital under surveillance, where he died of tuberculosis in 1938.

His Nobel medal is kept at the Carl von Ossietzky University of Oldenburg in Germany.