Japan has so far detained and questioned around 40 people, including journalists and academics, although many have been allowed to enter the country after several hours.
Police said one South Korean man was arrested after he shoved an airport immigration officer's head into a wall during questioning, angered by staff stopping him from taking pictures.
"They are classifying people unilaterally, without sufficient information or standards," Hong Hyung-suk, spokesman for the Korean Peasants League told Reuters by telephone from New Chitose Airport, where the group was being held. "We are not terrorists."
Some of the South Korean activist farmers, who also belong to the international Via Campesina peasant movement, have been arrested in the past and participated in protests against the World Trade Organisation in Hong Kong in 2005, representatives of the groups said.
Immigration officials in Sapporo were not immediately available for comment. Officials have previously confirmed that some people had been detained but declined to comment further.
Japan, fearful of violence during the July 7-9 G8 summit, has deployed 21,000 police officers on the northern island of Hokkaido, where the meeting will be held.
Police in Sapporo, the island's capital, conducted a security training exercise on Friday morning, stopping traffic and people as VIP black cars passed through the central streets of the city.
The security budget is some 30 billion yen (141.8 million pounds), topping the 113 million euros (89.5 million pounds) spent at the last summit in Germany.
DETAINED AND QUESTIONED
Other activists detained and questioned by immigration include political scientist Susan George, a vocal critic of the World Bank and International Monetary Fund, and Lydinyda Nacpil, the Asia-Pacific coordinator for Jubilee South, an advocacy group calling for debt cancellation for poor countries.
In some cases, detentions have stretched for over 10 hours, journalists, activists and academics have said. One academic said this week he was given no food or water for more than 12 hours.
"Japan, citing the G8 summit, has limited visas without specific cause and has insisted on extremely detailed plans from its visitors, making entrance into the country difficult," the G8 Summit NGO Forum, an umbrella group for non-governmental organisations, said in a statement.
"This is robbing us of free speech and the exchange of ideas," it said.
Two other South Korean nationals, one affiliated with the Korean Confederation of Trade Unions, have been deported, Japan's G8 Media Network has said.
Anti-G8 protests have become a fixture of G8 summits. On Sunday, two rallies in Tokyo gathered more than 1,000 people, including anti-capitalists, labour union members and protesters from abroad, such as Spain and South Korea. Eight men were arrested after scuffling with police at one of the rallies.
But tight security and the sheer cost of travelling to the remote site of the summit, at a hilltop luxury hotel in rural Hokkaido, is expected to dampen turnout compared with previous summits.
Demonstrations are anticipated near the summit venue -- where some 1,000-plus protesters are expected to gather in three camp sites -- and organisers of a peace rally in Sapporo ahead of the summit hope to draw 10,000 participants.
(Additional reporting by Jung Heejung and Yuriko Nakao in Sapporo and Edwina Gibbs in Tokyo; Editing by Brent Kininmont and Alex Richardson)
The Northampton Massive