Masked demonstrators hurled stones and flagpoles at police during a demonstration Saturday by tens of thousands of people against the upcoming Group of Eight summit in Germany, engulfing the harbor of this northern port city in chaos.

Officers in helmets and body armor at one point briefly retreated before a hail of stones, while other officers chased down fleeing protesters at the rally in Rostock in northern Germany.

Police marched in a line through a harborside street to scatter demonstrators, and were pelted with stones from behind. One of the organizers pleaded for calm from a loudspeaker: "The police are heading back so we can hold our protest in peace, that is what we want."

Cordula Feitchinger, a police spokeswoman, said one officer was slightly injured. She had no immediate total for arrests.

"There are massive assaults on police officers at the city's harbor right now," Feichtinger said. "The situation is currently very chaotic and we have to get it under control before I can tell you how many people have been arrested."

Feichtinger said one police officer was slightly injured but remained on duty.

The officially permitted march took place before the three-day summit beginning Wednesday where German Chancellor Angela Merkel hosts the leaders of the other G-8 nations — Britain, France, Japan, Italy, Russia, Canada and the United States. The leaders are expected to discuss measures against global warming, the fight against AIDS and poverty in Africa, and the world economy.

Earlier, a group of protesters attacked the hotel where an American delegation was supposed to stay during the G-8 summit this week, and some demonstrators also battered police cars with rocks, bottles and paint bombs, authorities said.

Police put the size of the demonstration at 25,000; organizers said it was 80,000.

The march began without violence, and most of the demonstrators remained peaceful.

But some demonstrators taunted members of the 13,000-strong police detachment from around Germany, and several hundred wore bandanas across their faces with sweat shirt hoods pulled down low to obscure their identities. They pulled up paving stones to hurl at police.

The protesters from around Europe and the world gathered at two locations early in the day for rallies, then marched in two groups along three-mile routes to converge on the harbor for the main demonstration — the biggest so far against the June 6-8 summit in the northern resort town of Heiligendamm.

Police with body armor and riot helmets lined the path through the city, and helicopters swirled overhead.

The protest was organized by several dozen groups under the motto "another world is possible."

"The world shaped by the dominance of the G-8 is a world of war, hunger, social divisions, environmental destruction and barriers against migrants and refugees," organizers said in leaflets handed out on the streets. "We want to protest against this and show the alternatives."

Dozens of different groups, including communists, anarchists and environmentalists, were taking part and messages were mixed: Some urged action from the G-8 countries in the fight against HIV/AIDS, African poverty and climate change, while others questioned the legitimacy of the existence of the G-8 itself.

Kay Stenzel woke at 3 a.m. to drive in from the eastern city of Bautzen with four friends to voice their discontent with the G-8 leaders.

"They want to impose their wills upon the poor nations," he said, waving a red flag emblazoned with a black cat — an animal he chose because it was "unruly."

On their Web site, organizers emphasized that they wanted a peaceful protest. They added, however: "This may be different with the actions following later in the week of protest."