A leading human rights group is calling for Malaysia to immediately release six leaders of an opposition party who are being held without charges because of their role in organizing a mass protest this month.

New York-based Human Rights Watch also called Thursday for charges to be dropped against 21 party members who are free on bail after being arrested in connection with the rally. It said the 21 are being prosecuted for exercising their rights “to freedom of expression, association and assembly.”

More than 20,000 people turned out for the July 9 demonstration in spite of government warnings that the gathering was illegal. A loose group of activists and opposition parties organized the rally to press for reforms to voting laws ahead of elections expected sometime next year.

The six members of the Parti Sosialis Malaysia were first arrested under a seldom-used law making it illegal to wage war against the king. They were released July 2 and immediately re-arrested under an Emergency Ordinance permitting indefinite detention without charges.

Human Rights Watch quoted lawyers for the six as saying they have been held in solitary confinement and are subjected to “continual and intense” interrogations. It said they have been blindfolded both in detention and when transported to meet their lawyers.

HRW said it is not yet clear whether police will allow the six to attend a July 22 hearing in Kuala Lumpur High Court to determine the lawfulness of their detention.