Ye Thein Naing and an unknown number of other protesters are being held at an improvised detention centre at the city's Kyaikkasan sports grounds, he said.
At least 100 people have been arrested since a series of anti-government rallies began on August 19 in protest at a huge increase in fuel prices, according to activists.
Myanmar has been rule by a military government for 45 years and displays of dissent are rare.Activist 'beaten'
"Ye Thein Naing was beaten and thrown onto a waiting truck by militia," said one activist who was at the protest.
"His leg was broken at that time, when he was beaten up. Even then, they still kept kicking him on the truck."
|"We have |
I do not understand why they treat us like rebels"
Su Su Nway,
and labour activist
Like many of the protesters, Ye Thein Naing is a member of the opposition National League for Democracy (NLD), headed by Aung San Suu Kyi, the Nobel peace prize winner who is currently under house arrest.
Su Su Nway, another NLD member and labour activist who led the march on Tuesday, said that she and other leaders of the protest movement have gone into hiding to avoid arrest.
"I heard the authorities have been watching my house all the time, waiting to arrest me," she said.
"We have only pens and books. I do not understand why they treat us like rebels.
"We are just making a statement on behalf of the people, including government staffers. We are sacrificing our young lives. I want the authorities to understand that."
The NLD won a landslide victory in general elections in 1990 elections, but the military has never recognised the result.Economic hardships
Meanwhile, a small rally of mostly NLD members was staged in the small town of Kyaukpadaung, located in a rural part of central Myanmar, activists said.
The protesters were jeered by supporters of the military government, and leaders were ushered into a meeting with the township chairman, who advised them of a ban on gatherings of more than five people before letting them go.