Church groups, environmental activists, cultural societies and feminists will all take part in the demonstrations, joining traditional anti-G8 protesters who say globalisation and capitalism perpetuate poverty in the developing world.
Organisers said makeshift camps on the outskirts of the harbour city, 200km north of Berlin, had filled up overnight and that packed buses and trains, some from the south of Germany, were flooding into the town.
There were no reports of trouble in Rostock overnight. The police presence was noticeable and officers were monitoring arrivals.
A police spokesman said: "We want to filter out those who are not interested in demonstrating, but who have just come to go on the rampage."
Organisers expect a larger, more boisterous crowd in Rostock after a series of police raids on left-wing activists and police orders to prevent them coming too close to Heiligendamm, a village 25km west of Rostock.
A 12-km security fence has been built around the resort where for the June 6-8 talks about climate change, aid and financial markets.
Eager to avert the violence that has accompanied past G8 summits, German leaders have issued pleas for peaceful demonstrations.
In 2001, a demonstrator was shot by police at a G8 meeting in Genoa. Since then, G8 summits have been surrounded by heavy security.
About 16,000 police officers are on duty in the week leading up to the meeting, Germany's biggest security operation since after the second world war.
Protesters are expected to block roads leading to the summit. They may also disrupt the arrival of delegates with their plan to blockade the nearby military airport at Rostock-Laage early next week.
Trouble may also flare after authorities refused permission for a demonstration by the far-right National Democrats (NPD) to go ahead in nearby Schwerin.
Organisers of the main anti-G8 demonstrations expressed concern that protesters who had planned to take part in the Schwerin rally might descend on Rostock instead.