On the night between the 12th and the 13th of August 1961, the police and units of the East German army began to close the border and, by Sunday morning, the border with West Berlin was closed. East German troops and workers had begun to tear up streets running alongside the border to make them impassable to most vehicles and to install barbed wire entanglements and fences along the 156 kilometers (97 miles) around the three western sectors, and the 43 kilometers (27 miles) that divided West and East Berlin. The barrier was built inside East Berlin or East German territory to ensure that it did not encroach on West Berlin at any point. Generally, the Wall was only slightly inside East Berlin, but in a few places it was some distance from the legal border. Later, the initial barrier was built up into the Wall proper, the first concrete elements and large blocks being put in place on 17 August.
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East Germany decided to upgrade the fortifications in the late 1960s to establish a "modern frontier" that would be far more difficult to cross. Barbed-wire fences were replaced with harder-to-climb expanded metal barriers; directional anti-personnel mines and anti-vehicle ditches blocked the movement of people and vehicles; tripwires and electric signals helped guards to detect escapees; all-weather patrol roads enabled rapid access to any point along the border; and wooden guard towers were replaced with prefabricated concrete towers and observation bunkers.