16 August 1961
The BBC reports from the scene as the Berlin Wall is constructed

Peter Woods reports from Berlin on witnessing the boundary between East and West being sealed with a concrete wall. He describes the ominous events that have taken place throughout the day, including the closure of an underground station and the movement of some East Berliners across the border. By the end of the night, it is likely that all access between the two sides of Berlin will be closed.

Construction of the Berlin Wall began in darkness during the early hours of 13 August 1961. As a result, many were unaware of what was happening. On 23 August, access between East and West Berlin was closed to all citizens. Temporary access by foreign visitors was permitted under controlled conditions. The barrier consisted of two walls with an area (or corridor) in between that was heavily guarded (a 'no man's land' or 'death zone'). A trench was also dug to prevent escape by vehicle. The walls were constructed not only of purpose-built concrete blocks topped with barbed wire or glass but also paving stones from the streets. Even a row of terraced houses formed part of the barricade, in Bernauer Strasse (Bernauer Straße).
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https://www.youtube.com/watch?v=fvou2RTHxVg