South Korean police are on high alert for "terror-like" protests being planned by a group of former navy commandos during next month's G20 summit in Seoul, the country's police chief said Monday.

National Police Agency chief Cho Hyun-Oh said that more than 100 veterans of the Underwater Demolition Team (UDT), an elite naval combat unit, plan "very violent" rallies to press their demands for bigger pensions.

"I am worried... They are trying to stage terror-like protests in the city centre," he told reporters.

He said some veterans were planning to set cars and oil tankers on fire near the summit venue in southern Seoul, where South Korea will host world leaders including US President Barack Obama for November 11-12.

"We will never tolerate this... and will respond sternly," said Cho.

Seoul will host world leaders including US President Barack Obama for the Group of 20 summit from November 11-12, in what is considered the nation's biggest appearance on the world stage since the 1988 Seoul Olympics.

Cho said last week the police would be on high alert against protesters planning to stage rallies around the summit venue, cautioning that South Korean protests "tend to be very violent and intense."

The association of UDT veterans said hundreds of its members planned to stage "major protests" by blocking maor roads with their cars and setting the vehicles on fire near the summit venue.

"Many of our members are well-versed in typical security tactics by the police and military, and they are ready to do just about anything," the association's spokesman told AFP.

"We have been begging for the benefit for years but have been utterly ignored by the government. We are left with no choice."

South Korean police plans to deploy a record 50,000 force to guard the event against possible threats from North Korea, international terrorists or violent protesters.

In addition to police, tens of thousands of troops will be deployed to key public facilities and mountain areas overlooking the summit venue, while naval forces and coastguards will monitor sea vessels.

Police have already drawn up strict security measures, including the creation of a special unit to protect the G20 leaders and surrounding the venue with security fences over two metres (seven-foot) high.

A special law also came into force this month giving police greater power to break up street rallies and allowing a military presence in public places.