Update 9:43 p.m. EDT: The justice minister said the government is still in place and praised the military for protecting security, Reuters reported.
"We declared a state of emergency, it's not a coup. Because of the situation, it's not stable, they kill each other every day," the general, who declined to be identified, told Reuters. "We need cooperation from them to announce to the people 'do not panic, this is not a coup.'"
He said soldiers were patrolling in Bangkok and had secured television stations.
The Australian government has issued a travel warning for its citizens traveling to Thailand following the declaration of martial law.
"We advise you to exercise a high degree of caution in Thailand due to the possibility of civil unrest and the threat of terrorist attack. You should pay close attention to your personal security at all times and monitor the media for information about possible new safety or security risks," the advisory read.
Update 8:05 p.m. EDT: Despite the Thai army's declaration of martial law, the caretaker government is still in office, an army spokesman told Reuters.
"This martial law is just to restore peace and stability. It has nothing to do with the government, the government is still functioning as normal," said Col. Winthai Suvari. An aide to the acting premier told Channel News Asia the government was not consulted about the martial law decree, but remains in office.
Update 7:16 p.m. EDT:
An announcement on Thailand's military-run TV said martial law had been invoked "to restore peace and order for people from all sides," asserting that the move "is not a coup," Agence France-Presse reported.
The army has also granted itself wide-ranging powers to enforce its decision, the BBC reported.
Thailand's army declares martial law in surprise move as country's political crisis intensifies, the Associated Press reports.
Earlier Monday, the acting prime minister was meeting with senators to search for a way out of the protracted political crisis, as anti-government protesters step up pressure to remove him and install a new administration, NDTV reported.
Thailand is stuck in political limbo following the dismissal of Prime Minister Yingluck Shinawatra and nine of her ministers on May 7 after a court found them guilty of abuse of power.
Commerce Minister Niwatthamrong Boonsongphaisan replaced her, but the anti-government protesters say he has no legal standing and want all remaining ministers of Yingluck's Cabinet to step down.
Developing story. Check here for updates.