The Thai military has declared martial law across the kingdom but denies it has staged a coup.
Troops moved swiftly through Bangkok after the pre-dawn announcement on Tuesday, securing key sites.
The army said it had made the move to restore order following six months of anti-government protests that have left 28 people dead, hundreds wounded and dragged down the Thai economy.
Military leaders said on-going mass rallies between political rivals "could impact the country’s security and safety of the lives and properties of the public".
"The public do not need to panic but can still live their lives as normal," a statement added.
The Bangkok Post reported that soldiers had entered several television stations in Bangkok and were taking up positions around the capital, including sites held by anti-government protestors.
Thailand has witnessed 18 coups in the past eight decades, the last of which ousted former prime minister Thaksin Shinawatra in 2006.
