Thousands of farmers and activists from around the world demonstrated Saturday in the northern Japanese city of Sapporo ahead of next week's summit of the Group of Eight rich nations.

Japan mobilised thousands of riot police to prevent any violence on the streets of Sapporo, the closest major city to the lakeside resort of Toyako, where world leaders will meet from Monday.

Dozens of masked protesters marching to rock music were warned by police against entering restricted areas as Japanese organisers of the rally called on the demonstrators to avoid violence and clashes with police. Organisers said two protesters were arrested.

"No violence! Please follow rules," said one of the organisers with a loudspeaker before the protesters hit the street. "Thousands of police have come here from throughout the nation and are watching us!"

Security was tight for the rally, which brought together union activists, anti-war demonstrators, farmers and students in a park in Sapporo.

Riot police wearing helmets and carrying shields patrolled downtown streets and the central park, part of a 21,000-strong force deployed to ensure security for the summit.

"Of course violence is not good," said a protester from London. "It does not cause any good."

"But look at the number of cops here," said the man, 50, who declined to be identified.

Organisers estimated 5,000 people took part in the rally.

Violent anti-globalisation rallies have marred past G8 summits -- last year militant activists threw Molotov cocktails and stones during demonstrations in Germany that drew tens of thousands of protesters.

Japanese authorities were taking no chances, refusing entry to 19 South Koreans, with others still detained at airports.

A speaker from the Korean Federation of Trade Unions deplored the move.

"We will not back down due to such suppression," he said to applause.

Ahead of the demonstration, around 100 farmers and fishermen waved banners and shouted slogans in the park, calling for the G8 to pay more attention to food producers.

"We should have a more balanced food supply in the world," said Japanese rice farmer Eiichi Hayashizaki, 69, holding a straw-woven banner saying, "Power to food producers!"

"Japan imports the majority of its food from overseas, so we don't starve ourselves. But the government should stop controlling rice production in the country."

Activists from charity Oxfam International warned of the impact of soaring food prices and climate change on world poverty as they performed their customary skit mocking the eight world leaders including Japanese Prime Minister Yasuo Fukuda and US President George W. Bush.

Wearing traditional Japanese kimonos, they sang a karaoke version of the ABBA song "Money, Money, Money."

"This isn't the time for a holiday, this is the time for sorting out problems," said Lucy Brinicombe of Oxfam International. "They shouldn't be distracted from finding solutions for the food crisis and climate change."

Global food prices have nearly doubled in three years, according to the World Bank, setting off riots in parts of the developing world.

Leaders of the eight major industrial powers -- Britain, Canada, France, Germany, Italy, Japan, Russia and the United States -- are reportedly set to agree on a new system of "food reserves" to assist hungry nations when they meet.