Parliament passed amendments in a vote in March, but further consultations and a second parliamentary vote were scheduled, and legislative elections were pushed back due to fierce backlash.
Those opposed to the changes fear they could allow further extensions of President Faure Gnassingbe's 19-year rule and his family's grip on power. His father and predecessor Gnassingbe Eyadema seized power in the coastal West African country via a coup in 1967.
In the second vote, lawmakers unanimously approved the amended charter under which the president will no longer be elected by universal suffrage, but by members of parliament.
The amendments also introduce a parliamentary system of government and shortened presidential terms to four years from five with a two-term limit.
It does not take into account the time already spent in office, which could enable Gnassingbe to stay in power until 2033 if he is re-elected in 2025, a highly likely scenario as his party controls parliament.
The amendments amount to a "project to ... confiscate power by a regime that is systematically opposed to any form of democratic change," a group of 17 civil society organizations said in a joint statement this week. They called on West Africa's main political and economic bloc, the Economic Community of West African States, ECOWAS, to take action in response.
In recent years, several other African countries, including Central African Republic, Rwanda, Congo Republic, Ivory Coast and Guinea, have pushed through constitutional and other legal changes allowing presidents to extend their terms in office.
The West and Central African region has also witnessed eight military coups in the past three years.
Faure Gnassingbe was last reelected in a 2020 landslide election which was disputed by the opposition.
The new constitution also creates a new role, president of the council of ministers, with extensive authority to manage government affairs.