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Barbed wire fences up to six feet (1.83 metres) high were put up during the night, and Berliners woke this morning to find themselves living in a divided city.
Train services between the two sectors of the city have been cut, and all road traffic across the border has been stopped.
Thousands of angry demonstrators quickly gathered on the West Berlin side of the divide. At one crossing point, protesters tried to trample down the barbed wire, only to be driven back by guards with bayonets.
The West German Chancellor, Konrad Adenauer, appealed for calm, saying in a broadcast to the nation this evening: "Now, as always, we are closely bound to the Germans of the Russian zone and East Berlin.
"They are and remain our German brothers and sisters. The Federal Government remains firmly committed to the goal of German unity."
There has been outrage from the international community at the abrupt decision to cut off one side of the city from the other.
A Foreign Office spokesman in London said the restrictions were contrary to the four-power status of Berlin, and therefore illegal.
The American Secretary of State, Dean Rusk, called it a "flagrant violation" of East-West agreements, and said there would be a vigorous protest to Russia.
The tide of people fleeing East Germany has grown to a flood in recent days, as the Soviet Union has taken an increasingly hard line over breaking away from the three Allied powers and forming a separate peace treaty with East Germany over Berlin.
Nearly 12,500 people left East Germany this week - over 2,000 more than the previous week.
The East German government has been taking desperate measures to stem the flow. Yesterday, border guards were intercepting trains near Berlin and interrogating passengers. Those who arrived in Berlin said only one in 10 was allowed through.
There had been rumours of a decisive crackdown on refugees since the East German parliament met yesterday and approved new, unspecified measures against them.
The rumours provoked an even more frantic exodus. Just before the borders were closed, the numbers more than doubled, with some 3,000 East Germans fleeing to the West in just 24 hours.
Within days troops began replacing the barbed wire with concrete blocks, and the wall became a permanent structure.
The concrete section eventually reached nearly 12 feet (3.6m) high and 66 miles (106km) long. There was a further 41 miles (66.5km) of wire fencing, as well as more than 300 watch towers.
Nearly 200 people died trying to cross the wall, and another 200 were injured.
As the Iron Curtain over Eastern Europe began to lift in 1989, thousands of East Germans found another escape route, via Hungary and Austria.
On 7 November 1989, the Communist government of East Germany resigned, and two days later a jubilant crowd tore down the Berlin Wall piece by piece.
The following year, East and West Germany were finally reunited.
The two architects of the Berlin Wall, East German leaders Erich Honecker and Egon Krenz, faced criminal charges over their actions.
Honecker escaped conviction due to ill health and died in exile in 1994. Egon Krenz was convicted of manslaughter, and sentenced to six-and-a-half years in jail. He was released early in December 2003 following an appeal.
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