Sri Lankan MPs have overwhelmingly approved proposals to let President Mahinda Rajapaksa seek an unlimited number of terms in office.
Constitutional amendments also hugely increase the president's powers, leaving him to appoint all the top judges and commissioners.
Critics say the changes could lead to dictatorship.
Demonstrators rallied both for and against the bill in the capital Colombo.
The main opposition United National Party boycotted the vote, but many of its MPs voted with the government.
Mr Rajapaksa is popular among the country's Sinhalese majority for presiding over the defeat last year of the Tamil Tiger rebels, after a 25-year separatist insurgency.