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BERLIN (Reuters) - Chancellor Angela Merkel invoked the injustices of communist East Germany on Friday to defend freedom and democracy during a visit to a notorious prison of the former Stasi secret police in Berlin.

Merkel, the daughter of a Protestant pastor who grew up in the German Democratic Republic (GDR), visited the ex-prison of Hohenschoenhausen a day before she launches her campaign for a fourth term as chancellor in a national election on Sept. 24.

Thousands of political prisoners were incarcerated in the jail, which after the fall of the Berlin Wall in 1989 and the 1990 reunification of Germany became a museum and memorial.

“The injustice that occurred in the GDR, that many people had to experience in an awful way, must not be forgotten,” said Merkel, who has just returned to work after a three-week summer holiday.

She said the visit to the former Stasi prison, two days before the anniversary of the start of construction of the Berlin Wall in 1961, was “of particular significance for me”.

“It seems a long time ago, but it warns us to work hard for freedom and democracy,” she said.

During her visit, Merkel met a former inmate, Arno Drefke, who often guides visitors through the spacious former prison, which is now preparing for a two-year renovation that will add new exhibition areas and seminar rooms.

Merkel and her conservatives, in power since 2005, are expected to win another term, although an opinion poll by Infratest dimap published late on Thursday suggested her popularity had dropped 10 percentage points to 59 percent.

However, Merkel appears to have little to fear as her main rival, the Social Democrats’ chancellor candidate Martin Schulz, saw his popularity hit a new low of 33 percent, down four points from last month.