(Reuters) - Following are key facts about the Berlin Wall, which was erected beginning in the early hours of August 13, 1961 and breached on November 9, 1989.
-- Faced with growing numbers of its citizens and skilled workers leaving the repressive state, East Germany's leaders feared a threat to their existence. Between the end of World War Two in 1945 and 1961, some three million people moved to the West from Soviet-occupied East Germany and East Berlin.
-- The 1,378-km (856-mile) frontier between East and West Germany, from the Baltic to Bavaria, had been sealed for a decade. That left Berlin a unique valve between them because of its special status under the four occupying powers - the United States, France and Britain as well as the Soviet Union.
-- Hardline East German leader Walter Ulbricht gave the order to build after winning backing from Soviet leader Nikita Khrushchev. The government entrusted the top-secret "Operation Rose" building project to Erich Honecker, who was to succeed Ulbricht as head of state 10 years later.
-- Shortly after midnight on August 13, the government ordered more than 40,000 East German soldiers and police to seal off all but 13 crossing points to West Berlin with barbed wire. The operation surrounded the 155-km (96-mile) perimeter of West Berlin, using more than 10,000 km (6,000 miles) of barbed wire.
-- The first concrete elements and large square blocks were used first on August 15, 1961. Within the next months the first generation of the Berlin Wall was build up: a wall consisting of concrete elements and square blocks
-- A second wall was built in June 1962 to prevent people escaping to the West. A concrete wall was added in 1965, which served until 1975 when the Grenzmauer 75 (Border Wall '75) was constructed, the final and most sophisticated version of the Wall.
-- The Grenzmauer was reinforced by mesh fencing, signal fencing, anti-vehicle trenches, barbed wire and over thirty bunkers. By 1989, the wall included 45,000 concrete blocks, 259 dog runs and 302 watchtowers.
-- Estimates of the number of people who died trying to escape vary. Berlin prosecutors say 169 people were killed between August 13, 1961, and November 9, 1989 -- 136 shot dead and 33 killed by mines. One group representing victims puts it at 239, a figure that includes many deaths that only came to light after the Wall fell. A recent study found that 1,065 people were killed trying to flee East Germany across all Cold War borders.
-- An estimated 5,000 people reached West Berlin. More than 75,000 people were imprisoned for trying to leave East Germany without permission.
-- The last East German to be shot dead crossing the border was Chris Gueffroy in February 1989. Winfried Freudenberg was killed in an escape attempt by balloon on March 8, 1989.
-- The "death strip" was a no man's land between the inner and outer segments of the Wall in East Berlin patrolled by soldiers ordered to use all means available to prevent people escaping. Border troops around Berlin opened fire 1,693 times between 1961 and 1989.
Writing by David Cutler, London Editorial Reference Unit; Editing by Sonya Hepinstall