Turkish women will be able to wear Islamic head scarves at university following a vote by the country's parliament.
Tens of thousands of secular Turks responded Saturday by demonstrating against the amendments.
"Turkey is secular and will remain secular," they chanted, many waving flags.
The demonstrators called for the government's resignation.
Turkey is predominantly Muslim but has been resolutely secular since Mustafa Kemal Ataturk founded the modern version of the country in 1923.
He banned religious clothing and changed the alphabet to Latin from the Arabic of the Koran.
The lawmakers voted 411-103 to amend the constitution to say that everyone has the right to equal treatment from state institutions and that no one can be deprived of (his or her) right to higher education."
Prime Minister Recep Tayyip Erdogan of the Justice and Development Party, which has ties to Islam, said Thursday, ""We will end the suffering of our girls at university gates."
He did say the law, while aimed at expanding democracy and freedom, would not permit students to attend class in full-length chadors or burqas.
President Abdullah Gul must sign off on the changes. However, he is an observant Muslim and is expectedly to approve the amendments.
Some lawmakers were vehement in their opposition to the change.
"This is a Black Revolution. The head scarf is a political symbol," said lawmaker Canan Aritman of the main opposition Republican People's Party, which said it would appeal the changes. "We will never allow our country to be dragged back into the dark ages."
Erdogan isn't trusted by secularists, in part because he tried to criminalize adultery. However, Turkey wants to join the European Union, and EU pressure forced Erdogan to backtrack on the move.
Many women fear that opening universities to head scarves could eventually lead to them being pressured to cover their bodies too.
With files from The Associated Press
