Hundreds of German police on Wednesday raided the offices and apartments of left-wing activists suspected of planning to disrupt next month's Group of Eight summit, leading security officials to tighten border controls ahead of the gathering. The crackdown was an attempt to ward off the violence that has marred past summits, particularly in 2001 in Genoa, Italy, when police and protesters clashed in the streets for days. Prosecutors said they were investigating more than 18 people suspected of organizing what they called a terrorist group that planned to carry out firebombings and other violent attacks. Some 900 federal and local police officers in cities including Berlin, Hamburg and Bremen searched about 40 premises used by several anti-globalist groups, they said. "The militant extreme left groups and their members are suspected of having founded a terrorist group, or of being members of such an organization, with the specific goal of staging fire bombings and other violent attacks in order to disrupt or prevent the upcoming G-8 summit in Heiligendamm," federal prosecutors said in a statement. Also Wednesday, Germany's interior ministry announced it would tighten border controls to screen out violent protesters. "As part of security measures for the G-8 summit in Heiligendamm, controls can be carried out on various border points... to prevent potential criminals and others intending to use violence from entering Germany," the ministry said in a statement. "Particular attention will be paid to potentially violent anti-globalization activists." People can usually travel freely throughout the European Union once they cross its common border, but individual countries maintain the right to tighten checks for security purposes. German security officials have built a â‚¬12.5-million (US$17-million) fence around the northern seaside resort of Heiligendamm, hoping to keep protesters away from the event, hosted by German Chancellor Angela Merkel. The leaders of the United States, Russia, Britain, France, Italy, Canada, and Japan are to attend. Last year's meeting in St. Petersburg, Russia, saw few protest, and those that were held were generally small, largely due to protesters' difficulty obtaining entry visas. But Germany is easier to reach from other European countries, and security officials do not want a repetition of the violence from Genoa in which one protester was killed by police. Federal prosecutors said they had focused on dismantling a particular server where they said many leftist groups and projects maintained their Web sites and mailing lists. Activists complained the raids were aimed at silencing legitimate protests against the G-8. Marches are planned - with official permission - in nearby Rostock ahead of the summit. "The only point of these searches is to criminalize and disrupt the protests against the G-8," the Anti-Fascist Leftists of Berlin said in a statement. "The accusation that terrorists would coordinate their movements through a leftist-run Internet server is ridiculous." In December, anti-G-8 activists splashed paint on a hotel in Heiligendamm and security officials in Hamburg cited a number of other lower-scale attacks, including several firebombings there and the torching of a car belonging to a top Finance Ministry official.