Three Hong Kong pro-democracy lawmakers were arrested and then released yesterday over their involvement in mass protests for free elections, the latest targets of a police probe campaigners say is tantamount to intimidation.

Police have vowed to investigate the "principal instigators" of the street blockades that ended in December when rally camps were cleared.

Albert Ho and Helena Wong, both of the Democratic Party, and lawmaker Charles Mok, voluntarily turned themselves in at Wan Chai police headquarters yesterday morning on the request of the police.

Ho and Wong held up small paper yellow umbrellas - the symbol of the democracy movement - as they went into the police station while supporters carrying umbrellas and placards shouted “We want universal suffrage".

Both Wong and Ho said the possibility of future charges remained.