"We all watched while a young man was beaten extremely hard and repeatedly by about three or four policemen in heavy riot gear," U Zarni, a spokesman for the Cooperative Committee for Trade Unions, told CNN.
Students are seeking changes to an education bill that they say limits academic freedom and have been in negotiations with authorities, but these have been curbed because of the crackdown.
The violence Tuesday occurred in Letpadan, a town of about 145 kilometers (90 miles) north of Yangon, the nation's largest city.
The students' march began in Mandalay, about 640 kilometers (400 miles) north of Yangon. Zarni said the students had been "given the red light" on proceeding to Yangon.
"They had decided they would drive into the city, meet with other students there for 15 minutes of talks and then disperse," Zarni said. "But within about an hour of being given this information, they were told that wasn't the case."
"Some students decided they would try, thinking the police would just take them away, but the police were so brutal."
Police arrested about 40 to 50 people, according to Zarni.
"They broke a reporter's camera and cheered about it. They hit women. They hit monks who were also there. They went into the monastery and detained everyone who had run to hide in there," Zarni said.
Human rights issues have been at the forefront in Myanmar, also known as Burma, as the country emerges from decades of authoritarian military rule, with elections planned for late this year.
"The public and we are losing confidence in the democratic transitions," Zarni said.
Khin Lay, director of the Triangle Women Support Group, agreed.
"Students, human rights, women's rights -- we're all extremely angry about the way we have been treated over the years," said Lay. "But we've been waiting for the 2015 elections with patience because we believe in the democracy movement, and we're hoping that we have the opportunity to exercise our right to vote. So they should show us some respect, too."