German leaders observe a minute of silence during a ceremony at the Bernauer Strasse memorial [GALLO/GETTY]

Germany is commemorating the 50th anniversary of the construction of the Berlin Wall with solemn ceremonies across the city and the opening of an expanded memorial for those who died trying to cross it.

The events began overnight on Saturday at a chapel, with a seven-hour-long reading about the lives of those killed while seeking freedom as they fled East Germany for West Germany.

At a sombre ceremony in Berlin, Mayor Klaus Wowereit, Chancellor Angela Merkel and President Christian Wulff paid tribute to the 136 people killed trying to get over the Wall.

Wowereit said the Wall, toppled in 1989, should serve as a reminder of freedom and democracy around the world.

Church bells peeled while trains and traffic came to a standstill at noon across Berlin for a moment of silence for the victims.

"We don't have any tolerance for those who nostalgically distort the history of the Berlin Wall and Germany's division," Wowereit said at the ceremony in front of a small section of the Wall recently rebuilt for posterity.

"The Wall was part of a dictatorship," he said. "And it's alarming that even today some people argue there were good reasons to build the Wall. No! There's no legitimate reason nor justification for violating human rights and for killings."

The Berlin Wall was erected in 1961 on the orders of East Germany's former leader, Walter Ulbricht, to stop a population exodus. In the 12 years since 1949, about 2.5 million people had fled East Germany.

Divisions remain

On the morning of August 13, 1961, East Berliners woke to find soldiers had blocked off the streets, cut off rail links and begun building a wall of barbed wire and cemented paving stones, which over the years grew in height and eventually stretched over 155km.

The sudden move by communist East Germany to close the border shocked the world and ripped a hole through Berlin that gaped for more than 28 years until the wall finally fell on November 9, 1989, in a bloodless uprising.

At least 136 would-be refugees are known to have died at the Berlin Wall. Historians suggest that the overall figure of those killed while fleeing from East Germany stands at between 600 and 700. Victims' groups say the number is higher still.

Germans note that the division still lingers between the poorer east and the more prosperous west.

"There's still a big wall," Klaus Schroeder, a political scientist at Berlin's Free University, told the AFP news agency.

"Most east Germans don't know which world they belong to. They don't see themselves as full members of a unified Germany."