Army chief Prayuth Chan-ocha warned that troops, which took control over security responsibilities across the country, would take action against anyone who used weapons and harmed civilians following longstanding anti-government protests that left 28 people dead and hundreds wounded.
"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.
Both pro- and anti-government protesters are camped out at different places in the capital Bangkok and to prevent clashes, the army warned them against marching, Al Jazeera reported.
An announcement on military-run television said that the imposition of martial law was not a coup and affirmed that the caretaker government was still running the country
"The public do not need to panic but can still live their lives as normal," it added.
Thailand's caretaker prime minister Niwattumrong Boonsongpaisan issued his own plea for harmony in the country, telling reporters the government wanted "peace and order to prevail in the country".
Martial law allows the military to take over power from the police and other state authorities. The army can now search people, homes, vehicles and even open letters and messages.
Troops, some in jeeps mounted with machineguns, stopped some traffic from entering Bangkok after the martial law order. They also took up position at intersections and secured television stations, but life went on as normal in most of the city.
Critics accused the army of media censorship after it ordered 10 satellite TV channels, both pro- and anti-government, to stop broadcasting.