Leaders of the 10-nation Association of Southeast Asian Nations (ASEAN) signed the joint declaration, claiming it would enshrine human rights for the region’s 600 million citizens.
"It's a legacy for our children," said Philippine Foreign Secretary Albert del Rosario but Human Rights Watch's deputy Asia director Phil Robertson was scathing.
"Our worst fears in this process have come to pass," said Robertson.
"Rather than meeting international standards, this declaration lowers them by creating new loopholes and justifications that ASEAN member states can use to justify abusing the rights of their people," Robertson added.
More than 60 rights organizations had called for the agreement to be postponed, pointing to ASEAN's diverse political systems, ranging from freewheeling democracy in the Philippines to authoritarian regimes in Laos and Vietnam.
Campaigners had also cited a lack of transparency, saying there had been inadequate consultation while the text was being drafted.
ASEAN secretary-general Surin Pitsuwan said a "safety net" amendment had been inserted on Saturday to read that ASEAN members would "implement the declaration in accordance to the international human rights declarations and standards."
"This certainly can be used to monitor the practice, the protection, the promotion of human rights here in the ASEAN countries," Surin said.
"It's an important benchmark for ASEAN to be kept honest in terms of its human rights obligations," said Indonesian Foreign Minister Marty Natalegawa.
ASEAN invites guests
President Barack Obama, who on Sunday began a three-nation Asian tour in Thailand, is due to attend an expanded ASEAN gathering in Cambodia on Monday alongside guests including the leaders of China, India and Australia.
The South China Sea dispute between various Asian nations, including China, is expected to be high on the agenda.
According to the president's aides, Obama will also express "grave concerns" about Cambodia's rights record and press the need for political reform at a meeting with Cambodian Prime Minister Hun Sen.
sej/ipj (AFP, dpa, dapd)