Updated

SEOUL, South Korea -- Police have arrested four people who scuffled with riot police during anti-globalization rallies ahead of this week's Group of 20 summit, a police officer said Monday.

About 20,000 people rallied at a large plaza near Seoul City Hall on Sunday, just four days before South Korea hosts a two-day gathering of leaders from the G-20 advanced and developing economies.

The rally initially was peaceful, but some protesters scuffled with riot police who tried to prevent them from marching through nearby downtown streets. Riot police fired pepper spray at some protesters at the front of the crowd, forcing them back.

Four men were arrested during the scuffle and were under investigation Monday, a Seoul police officer said on condition of anonymity citing department policy. He gave no further details

The protesters oppose globalization and say the G-20 is not focusing on creating jobs or protecting social programs.

Labor activist Lee Chang-geun accused the G-20 of failing to formulate meaningful measures to curb speculative financial capital and of pushing cuts in public spending on social welfare.

Activists plan to stage another major rally to mark the start of the summit on Thursday, he said.

In Manila, six Filipino activists who planned to join anti-globalization rallies in Seoul said they were denied entry by South Korean immigration officials on Saturday and forced to return home.

"We condemn the South Korean government and we demand that it apologize to us and remove us and others from its blacklist," Josua Mata, one of the activists, told The Associated Press in Manila.

Philippines presidential spokesman Herminio Coloma said Monday that the Philippine government will consider filing a diplomatic protest over the activists' expulsion, after hearing from its diplomats.

South Korea's Justice Ministry said the denial was based on a law that bans the entry of people who could undermine national interests and social order. It gave no further details.

The G-20 forum, founded in 1999, has taken on greater significance since the global financial meltdown and is seeking to reform the world economy to ensure stable growth and strengthen the financial system to prevent a recurrence of the 2008 crisis. It held its first summit in Washington two years ago.

South Korean police and military are on heightened alert to prevent any attempt by protesters, terror groups and North Korea to sabotage the summit, which will include President Barack Obama and other world leaders. North Korea has a history of staging provocations when world attention is focused on rival South Korea.

The Korean peninsula remains officially at war because the 1950-53 Korean War ended with an armistice, not a peace treaty.