Sri Lankan MPs have approved proposals to let President Mahinda Rajapaksa seek an unlimited number of terms, in a move critics say could lead to dictatorship.
The constitutional amendment also hugely boosts the president's powers.
Amid protest and counter-protest in the capital Colombo, MPs in the 225-member parliament voted for the proposals by 161 to 17 against.
The main opposition United National Party boycotted the vote, but many of its MPs voted with the government.
The lifting of the two-term presidential limit allows Mr Rajapaksa to stand again in 2016.
'Bribes and threats'
The amendment also empowers him to appoint all the top judges and commissioners for elections, human rights and other affairs, unfettered by any legal veto.
The BBC's Charles Haviland in the capital Colombo says some opposition supporters accuse the government of using bribes and threats to secure MPs' votes, which the administration denies.
The administration had been expected to easily win on Wednesday the two-thirds majority needed to pass the amendment.
Opposition and human rights groups questioned why the government had pushed the amendment through the supreme court and parliament in barely 10 days, declaring it an urgent matter.
Civil society activists called Wednesday a "black day" and asked Sri Lankans to wear that colour.
Mr Rajapaksa is popular among the country's Sinhalese majority for presiding over the defeat in May last year of the Tamil Tiger rebels, after a 25-year separatist insurgency.
Supporters of the president, who was resoundingly re-elected for the second time in January, said the measures would strengthen democracy in Sri Lanka.
Foreign Minister GL Peiris told parliament: "Political instability is the last thing we want in Sri Lanka as we seek to derive the fullest benefit for our people from the unique opportunity we have today having eradicated terrorism."
But Sarath Fonseka - the general who led the army to victory against the Tamil Tigers before falling out with Mr Rajapaksa, then unsuccessfully challenging him at the ballot box in January - said it was "the last nail into the coffin of democracy".
"The president is trying to take the country towards dictatorship and we will fight against it," he said.
Mr Fonseka is a prisoner of state but, as an elected MP, able to attend parliament.
He has been dishonourably discharged from the army and is being tried for alleged corruption, but he says he was victimised simply for daring to stand against Mr Rajapaksa in elections.
Critics accuse the president of trying to set up a family dynasty - his son is an MP, and three of the president's brothers are in top positions.