SEOUL (AFP) - Angry protesters and relatives of South Korea's ferry disaster victims clashed with riot police on Friday after they were blocked from marching toward the president's office in Seoul.

Violence erupted when some 2,000 protesters, led by dozens of relatives wearing yellow jackets, tried to force their way through police barricades.

Some of the protesters kicked and punched riot police, who turned water cannons on them.

Scattered clashes lasted for more than an hour, with the protesters chanting slogans accusing the government of seeking to hamper a probe into the disaster.

In separate clashes on Friday - May Day - police turned water cannons on workers protesting for labour rights who were also trying to march to President Park Geun Hye's office.

The Sewol was carrying 476 people when it went down off the southwest island of Jindo on April 16 last year. Of the 304 who died, 250 were pupils from the same high school.

The tragedy shocked and enraged the country as it became clear that it was almost entirely man-made - the result of an illegal redesign, an overloaded cargo bay, an inexperienced crew and an unhealthy nexus between operators and state regulators.

The government last week announced plans to raise the sunken ferry to the surface following a series of protests that have seen clashes with police.

The announcement met one of the key requests from the victims' families, but they have continued protests to push their demand for a fully independent inquiry into the sinking.

On Tuesday, an appeals court found the ship's captain guilty of murdering those who died in the disaster and upped his sentence from 36 years to life imprisonment.

Fourteen other crew members were given prison terms ranging from 18 months to 12 years.