Police arrested four people as more than 1,000 marched in northern Japan on Saturday to protest against the Group of Eight summit taking place near the city of Sapporo.
Marchers criticized globalization, which they blame for the world's soaring food prices, dependence on fossil fuels and global warming.
Thousands of riot police were deployed along the march route. Police clashed with demonstrators after they tried to squeeze people into one lane as the rally moved from a park onto the streets of Sapporo.
Some protesters wore masks of leaders from the top industrialized countries and carried a banner reading, "G8: Don't wait till it's too late."
The three-day summit is for leaders of the United States, Canada, Japan, Russia, France, Britain, Italy and Germany.
Prime Minister Stephen Harper planned to leave Canada on Saturday for the city on the island of Hokkaido. The meetings will take place at a nearby hot springs resort.
Harper has previously argued that it would be useless to reach a global agreement on climate change unless it includes the world's major polluters.
Japan had put climate change at the top of the summit's agenda, but it could be overshadowed as leaders discuss the global credit crunch and inflationary pressures on food and fuel.