The army's commander, General Prayuth Chan-ocha said the military had stepped in to restore order, and invited both sides of the kingdom's bitter political divide to talks.
"We ask all sides to come and talk to find a way out for the country," Prayuth told reporters after meeting directors of government agencies and other high-ranking officials.
Prayuth warned pro- and anti-government protesters to avoid violence, and said martial law would be revoked after peace and order had been restored. Earlier, the military insisted the intervention was not a coup d'etat and said in a statement: "The public do not need to panic and can continue on with normal life."
The statement – read out in one of a number of televised proclamations – said the military had been compelled to act because "groups with bad intentions" risked escalating a six-month-long political crisis by threatening to overthrow the elected government.
"The royal Thai army intends to bring back peace and order to the beloved country of every Thai as soon as possible," said Prayuth in the statement, adding: "We intend to see the situation resolved quickly."