"Alarms, wires, clocks and bangers" were uncovered during raids targeting 21 people suspected of "seriously trying to disrupt or prevent," the June 6 to 8 summit in the Baltic Sea resort of Heiligendamm, German weekly news magazine Focus reported.

In an issue due to appear on Monday, it said police found "non-conventional explosives" at a farm near Berlin. Fake identity cards were found at the home of another suspect in the northern city of Hamburg.

Berlin is expecting up to 100,000 left-wing protestors to demonstrate at the meeting of leaders of the Group of Eight most industrialized nations.

Authorities are planning to deploy 16,000 police in Heiligendamm and the nearby city of Rostock and put up a 12-kilometre-long (7.5-mile-long) security fence.

German magazine Der Spiegel weekly said several retired people were on the list of the 21 suspects, adding that the oldest among them was 68.

"A show of force"

On Wednesday, 900 police officers raided 40 sites in northern Germany linked to left-wing activists believed to be preparing arson attacks and other violent protests during the summit.

The massive security operation has been slammed by critics as overblown and an unnecessary show of force by the German state.

Peter Ritter of the opposition Left Party told news agency dpa on Sunday said the government by deliberately criminalizing G8 opponents was also discrediting their criticism of the policies of industrialized nations.

"If anti-globalization activists are collectively painted as being violent, then it's a statement on their goals which are shared by many people," Ritter said.

In an editorial on Sunday, the Zurich-based SonntagsZeitung said the German raids on left-wing bookshops and offices was "a show of force by the German state." The paper added that the move had only served to fuel discontent among left-wing groups and bolster their readiness to protest violently.

In response to the raids, more than 5,000 protestors later took to the streets of Berlin, Hamburg and smaller German cities.

Anti-globalization group Attac said on Friday the police clampdown had only served to encourage protestors. "One of the consequences of the police operations was to mobilize people," Peter Wahl from Attac told ZDF television.

Critics incensed by security measures

What has also angered critics is a controversial proposal by Interior Minister Wolfgang Schäuble to place potential troublemakers in preventative custody.

Schäuble said potential troublemakers could be detained for up to two weeks if there were firm indications they are planning criminal actions. In Germany, people can be put behind bars for 14 days if police find evidence that they are planning a crime. The government threatened to do this with hooligans during the soccer World Cup in Germany last summer.

The minister has already announced that Germany was re-imposing border checks to stop potential offenders, mainly opponents of globalization, from entering Germany to protest at the summit.

Over the weekend, Schäuble said he was not responsible for the recent nationwide raids on left-wing groups -- a decision, he said, was taken by the federal state prosecutor -- but he defended the measure.

"I don't have the slightest doubt that the measures were absolutely right," Schäuble said on the sidelines of an EU interior minister meeting in Venice on Saturday.

Police expert warns against ignoring other threats

Schäuble said Germany expected an increased security threat during the summit. He said the fact that Germany was not subject to terrorist attacks during major events such as last year's World Cup "does not mean we will be spared this time."

Fears about security were raised when opponents of the summit claimed responsibility for torching a car on the property of a German finance ministry official in the northern city of Hamburg in December. Protesters have also splashed paint on the summit hotel.

However, some have warned against focusing excessive attention on the threat of violent left-wing demonstrations and threats in the run-up to the summit.

"We shouldn't underestimate the immense dangers posed by Islamic terrorists," said Konrad Freiberg, head of the police union, in an article for the Bild am Sonntag paper. "It was while the G8 summit was taking place in Gleneagles in Scottland in 2005 that bombs planted by Islamic terrorists exploded in the London subway."

Freiberg also warned that police forces would be overstretched during the security operation for the G8. Freiberg called the G8 summit a "Herculean task" and said it would constitute the "biggest police operation of all times."

Several G8 summits in recent years have been scarred by violence, most notably the meeting in the Italian city of Genoa in 2001 when an anti-capitalist protester was killed by police during riots.

Demonstrations at recent G8 summits have been kept well away from the venue, although violence has broken out in nearby towns.