A weekend of planned vigils and marches to mark the one-year anniversary of the deadly Sewol ferry sinking in South Korea has turned into tense clashes between demonstrators and police.

Seoul police say they put 12,800 riot officers on the streets to manage demonstrations by families of Sewol victims and their supporters, who are demanding a thorough investigation of the ferry disaster that killed 304 people. More than 250 victims were students from a single high school.

Since the disaster, a promised outside investigation into the sinking has yet to begin, nine victims are still unaccounted for and the ferry remains at the bottom of the ocean.

In Gwanghwamun Square, the symbolic city center where demonstrations have been taking place on-and-off since the April 2014 ferry sinking, a planned sit-in and march to the Blue House, the home of the president, were prevented by police who say demonstrators were illegally protesting. As crowds swelled, officers deployed water cannons to keep demonstrators away from police lines. On Friday and Saturday nights, police used pepper spray on crowds during tense moments — a move Amnesty International is calling an "insult to victims" and illegal.

As of Saturday afternoon local time, police say 22 people have been arrested in the tensions.

Using buses to create a tight perimeter around the heart of central Seoul Saturday, police successfully kept inside the demonstrators, who are in their third day of events commemorating the sinking, and kept out regular citizens trying to move about a normally busy, tourist-filled part of the city.

Police also shut down major arteries for traffic and a central subway station, bringing commerce to a halt in South Korea's capital.

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