None of the peoples of the former Warsaw Pact countries were as highly monitored as the East Germans: there was one Stasi officer for every 180 people, compared with one KGB officer for every 595 people in the Soviet Union.

Every aspect of people's lives, but especially any contact with abroad, was secretly noted and recorded. The information could range from political sympathies to the most banal details, with the aim of gathering material to discredit anyone the regime wanted to sideline.

Damaged Stasi documents

In January 1990, two months after the Berlin Wall fell, the agents realized the game was up and began destroying their life's work, shredding and burning files and then desperately tearing them up once the shredding machines broke down. It was the people's suspicions that the files were being destroyed which triggered the storming of the headquarters of the State Security Service, to give the Stasi its full name, on January 15.

The invasion of the complex of almost 50 buildings on Normannenstrasse in East Berlin sent the message to the East German leaders trying in vain to save their crumbling regime that the people would not allow the Stasi to be relaunched in another form, with a more 'acceptable' face.

Heinz Maier was among the first demonstrators to enter the headquarters. Then aged 51 and a project engineer, he was driven to action for two reasons. "The first was personal. My son was in a Stasi jail for trying to flee the country," Maier recalled this week.

"The second was political -- this huge security apparatus had to be brought down and, above all else, we didn't want the files to be destroyed," he said, explaining how on the evening of January 15, the doors of the building were locked and it appeared closed, but within 15 minutes thousands of people had forced them open and we stormed in.

"The lights were on in one of the buildings and I could see the windows were open on an upper floor. It was clear that files were being destroyed up there," he said. The next day, Maier joined one of the citizens' committees set up by the demonstrators to decide the way forward. "I was one of the first civilians to be allowed in to see the files. That was when the scale of what the Stasi had been doing hit me full in the face."

Massive amount of files

What he saw were millions and millions of files, each numbered and filed. If all the files complied by the Stasi were laid side by side, they would stretch for 180 kilometers (112 miles). The system had never been computerized, so there were also 17 million index cards.

On a tour of the buildings this week, journalists were shown a fraction of the files, and one example of the work of the so-called 'M-section' agents who monitored and often confiscated the mail sent from East Berlin. Accompanying a file with the name blanked out for legal reasons, there were careful notes of the letters sent, what they contained, and even photographs of the letters.

The Stasi archive has thousands of files.

The storming of the Stasi headquarters led to the creation in August 1990 of a state-run agency to manage the files, which opened them up to the public to allow people to see what the spies had written about them.

But the future of the agency is now threatened. A leaked government report last year suggested the collection should eventually be broken up and stored in the federal archives in Koblenz, in the former West Germany.

The current head of the Stasi Data Authority, Marianne Birthler, a former East German pro-democracy activist, says people want the files to be kept open. Over one million people have already seen their file and just under 94,000 people applied last year alone to read theirs, which can often be a daunting experience.

"Some people have only just plucked up the courage," Birthler said.