The national parliament voted in favour of the 18th amendment to the 1978 constitution on Wednesday, scrapping a two-term limit on the presidency and handing sweeping new powers to President Mahinda Rajapakse.
The law grants the head of state the ability to appoint officials to key posts in previously quasi-independent institutions such as the judiciary, police and election commission.
"The constitution was used to consolidate power, without rebuilding governance structures," said Paikiasothy Saravanamuttu, executive director of the local thinktank the Centre for Policy Alternatives.
"It's a setback for democracy. Parliament is now a rubber-stamp institution, It's a dark period for us," he added.
The reforms were debated on Wednesday and passed with 161 in favour and 17 against in the 225-member parliament. The main opposition United National Party (UNP) boycotted the debate, saying it did not want to be "contaminated.
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Government ministers argued that the changes were needed to