Riot police in Mexico City have clashed near Congress with students opposed to the government's education reforms.
Some protesters threw rocks and firebombs to disrupt the opening of a new legislative session of parliament and police responded with tear gas.
The violence came after two weeks of protests by teachers against mandatory evaluations for their performance.
Congress is due to vote on the measure, which the government says will improve the quality of public education.
Their faces covered, protesters shouted "Out with Pena Nieto", referring to Mexico's president.
A number of them were arrested for allegedly attacking police officers.
Meanwhile, tens of thousands of teachers held a separate, peaceful protest against the education reforms.
But they were prevented from reaching the Congress building by thousands of riot police and military officers.
'No turning back'
President Enrique Pena Nieto enacted a major overhaul of the education system in February, aimed at centralising the process for hiring, promoting and retaining teachers.
Lawmakers still have to vote on one controversial secondary draft law which would force teachers to undergo compulsory performance tests.
Striking teachers fear the new measure could lead to massive lay-offs, but the president has vowed not to turn back.
The protests have forced the cancellation of many classes across Mexico, and the postponement by a day of President Pena's first state of the nation address.
He was originally scheduled to address the nation on Sunday.