The Ukrainian Security Service has said that a terror attack has been prevented at a volunteer center in Kharkiv.

"We have detained a group of persons as they were preparing a terror attack at a volunteer center," Oleksandr Pyvovar, head of the Kharkiv department of Ukraine's Security Service, told the press.

Four people, all of them Ukrainian citizens, have been detained, he said.

The suspects confessed that they had also been preparing a terror act at a shopping center, he said.

The Ukrainian Security Service earlier said that several persons suspected of setting off a bomb during Sunday's march in Kharkiv, had been detained. The bombing left two people killed and nine wounded.