A reporter for TNN television said electricity went out in the Red Shirt protest zone in Rajprasong, a posh area of shopping malls, hotels and upscale apartments that they have occupied since April 3 in the intractable political crisis gripping the country.
Calls to police and army spokesmen seeking comment were not answered.
The Red Shirts, many from the rural poor, are demanding an immediate dissolution of Parliament. They believe Prime Minister Abhisit Vejjajiva's coalition government came to power illegitimately through manipulation of the courts and the backing of the powerful military.
Abhisit offered to hold elections on Nov. 14, but withdrew the offer Wednesday after the Red Shirts refused to disperse, insisting that the deputy prime minister be held responsible for clashes between protesters and security forces on April 10 in another part of Bangkok. The fighting left 25 people dead. Another four were killed in subsequent violence.
Earlier Thursday, the government announced a lockdown on Rajprasong and warned that troops will not hesitate to shoot armed "terrorists."
"During the closing-in operation today, it's possible that terrorists in the area would move in and encounter the officers. Therefore, the authorities must execute measures according to international standards and rules of engagement. Live ammunition will be used," said Col. Sansern Kaewkamnerd, the spokesman of the agency charged with ending the protest.
Armored personnel carriers and snipers were to surround the so-called Red Shirt protesters who have barricaded themselves behind piles of tires and bamboo spears in the 1-square-mile (3-square-kilometer) area, Sansern said.
Leaders of the Red Shirts were defiant, saying their supporters would never "surrender" and were not afraid to die.
The Red Shirts, who are largely drawn from the rural and urban poor, see Abhisit's government as serving an elite insensitive to the plight of most Thais. The protesters include many supporters of former Prime Minister Thaksin Shinawatra, a populist leader who was accused of corruption and abuse of power and ousted in a 2006 military coup.
Thaksin, a former telecommunications billionaire who fled overseas to avoid a corruption conviction, is widely believed to be helping to bankroll the protests. He claims to be a victim of political persecution.
Associated Press writers Jocelyn Gecker, Vijay Joshi and Grant Peck contributed to this report.