Soifah Osukonthip, an NBT newscaster, said the protesters repeatedly shouted at staff to "get out".
"All 150 of us were rounded up for a short while before police arrived and talked them down to the ground floor," Soifah said.
The People's Alliance for Democracy (PAD), an anti-government protest group, initially denied any involvement in the raid but after the arrests PAD leaders at a semi-permanent protest venue in central Bangkok called on their followers to march on the state broadcaster.
Hundreds of protesters were seen bursting through police lines and protesting outside the gates of the media compound.
Thousands more were gathered outside the compound of the main government offices, Government House, preventing employees from entering the compound.
"We are now controlling most of the key government offices to prevent them from coming to work," Sondhi Limthongkul, a PAD leader, said on Tuesday.
The PAD is hoping to draw "hundreds of thousands" of people to Tuesday's rally outside Government House, in its latest attempt to unseat the elected government.
Police say they are expecting only up to 35,000 people but do not plan to confront the demonstrators, a spokesman said.
But Samak Sundaravej, the prime minister, said in a televised speech on Monday that he was not threatened by the rally and that "if they break any law, they will be dealt with accordingly".