Hundreds of protesters clashed with police Monday ahead of this week's Group of Eight meeting, as anti-globalization activists challenged attempts by security officials to keep them away from the summit town of Heiligendamm.

About 800 protesters demonstrated outside Rostock's immigration office demanding "global freedom of movement and equal rights for all," including refugees and asylum seekers. After a peaceful start, the demonstration turned violent and "some protesters started to throw bottles at officers," police spokesman Lyder Behrens said.

The Web site Spiegel Online reported that a photojournalist was injured and four people were detained when 400 demonstrators clashed with police. Police could not immediately confirm the report.

Another 20,000 demonstrators were expected to hold an anti-G-8 rally in downtown Rostock later Monday.

On Saturday, around 3,000 black-hooded anti-G-8 protesters had pelted police with rocks and bottles in Rostock. Authorities said more than 400 officers were injured, 30 of whom were hospitalized with broken bones and cuts. Organizers said 520 demonstrators were hurt, 20 of them seriously. More than 2,000 protesters were still in Rostock on Monday, police said.

The situation around Heiligendamm was calm Monday ahead of the three-day meeting starting Wednesday, when German Chancellor Angela Merkel will host the leaders of Britain, France, Japan, Italy, Russia, Canada and the U.S. for discussions on issues including global warming, aid to Africa and the world economy. President Bush left Monday for the summit, with his first stop in Prague, in the Czech Republic.

Germany is determined to avoid a repeat of the debilitating violence that has marred previous G-8 summits, notably in Genoa, Italy, in 2001, where one protester was killed. It has reinforced its border controls ahead of the summit.

Anti-globalization activists have complained that security surrounding the June 6-8 summit is excessive.

Germany's Constitutional Court said Monday that an alliance of activist groups had challenged a lower court's ban on protests outside of Heiligendamm.

That ban came into force last week, when public access to Heiligendamm was shut off. Authorities had announced earlier that starting on Wednesday the demonstration ban will be expanded to about 3 1/2 miles beyond a 7 1/2-mile fence that was built around Heiligendamm. It was not clear when the court would rule on the case.

The government said 85 people had been refused entry to Germany ahead of the summit and that 15-20 percent of those who were detained temporarily during Saturday's riots in Rostock had been foreign nationals.

Merkel, in an interview on the n-tv news channel Sunday, deplored the "terrible, dreadful pictures" of violent protesters clashing with police. "Violence is no way to solve things and shows that the police methods are necessary," she said.

Merkel defended security measures such as pre-summit raids on the offices of protest groups, some of whom have vowed to try to disrupt the summit by blocking roads leading to Heiligendamm. She has also said, however, that peaceful protesters have every right to make their point.