The UN joined Burmese opposition groups, Western government officials and international rights organizations in calling for the Burmese military government’s immediate release of three prominent activists on Thursday.

In a released statement, UN Special Rapporteur Paulo Sergio Pinheiro called for the release of Min Ko Naing, Ko Ko Gyi and Htay Kywe-three members of the 88 Generation Students Group who have been in police custody since Wednesday, when they were taken from their homes by police and later held in Special Branch offices at the Ministry of Home Affairs.

“The arrest leads to serious questions regarding the will of the government of Myanmar [Burma] to resume an effective dialogue with the various stakeholders who should be associated with the National Convention,” Pinheiro said. In his latest report, which was submitted to the new UN Human Rights Council on Wednesday, Pinheiro highlighted the lack of political development in Burma, including the continued detention of more than 1,000 political prisoners in the country’s numerous prisons.

Although the reason for the arrests-as well as formal charges-remain unknown, the move coincided with the 18th anniversary of the founding of the main opposition party National League for Democracy, which the activists were scheduled to attend. The NLD on Friday also issued a statement calling for the immediate release of the three activists.

“They are helping national reconciliation, which is much needed in the country to solve the current political crisis,” said Nyan Win, an NLD spokesperson. “The act is contrary to the government’s claim to commitment to democratic reform in the country.”

State-run media reported in August about the activities of the trio as inappropriate acts of “political agents,” which included their meeting with the new British ambassador.

The 88 Students Generation group-comprising former political prisoners and student activists-on Thursday urged the military regime for an explanation of the detention of its leaders and for their immediate release. Some members of the group have begun planning a signature campaign for the freedom of their detained leaders if authorities fail to release them by Sunday, according to the group.

Several other rights groups and activists, including the London-based Amnesty International and New York-based Human Rights Watch, condemned the arrest and called for their freedom.

The UN statement said that Pinheiro “is extremely worried” with the regime’s latest move as he has received numerous reports of rights abuses in pre-trial detention over the past seven years. Meanwhile, the UN Security Council is to discuss Burma on Friday. The 15-member body voted 10 to four in favor, with one abstention, to put the issue of Burma on the council’s formal agenda on September 15.