Thailand's military rulers say they are monitoring a new form of silent resistance to the coup – a three-fingered salute borrowed from science fiction blockbuster The Hunger Games – and will arrest those in large groups who ignore warnings to lower their arms.

The raised arm salute has become an unofficial symbol of opposition to Thailand's 22 May coup, and a creative response to several bans the ruling junta has placed on freedom of expression.

"At this point we are monitoring the movement," said Colonel Weerachon Sukhondhapatipak, a spokesman for the ruling junta. "If it is an obvious form of resistance, then we have to control it so it doesn't cause any disorder in the country."

Since staging its bloodless coup, the military has prohibited political gatherings of more than five people and tried to enforce a ban on criticism of the coup by closing politically affiliated television stations and blocking hundreds of websites.

On Sunday, authorities deployed nearly 6,000 soldiers and police in Bangkok to prevent planned protests against the coup. Amid the heavy security, small flash mobs appeared in a central shopping district where the salute was unveiled for the first time.

Asked what the symbol meant, some protesters say it stands for the French revolution's trinity of values: liberty, equality, fraternity, while others say it means freedom, election and democracy. A photo montage circulating online paired a picture from The Hunger Games with a graphic of three fingers labelled, 1. No Coup, 2. Liberty, 3. Democracy.

In the book and movie series, the salute is a symbol of rebellion against totalitarian rule and stands for thank you, admiration and goodbye to someone you love.

"We know it comes from the movie, and let's say it represents resistance against the authorities," Weerachon said, noting that if authorities encounter the salute they will first ask protesters to stop.

"If a single individual raises three fingers in the air, we are not going to arrest him or her," he told the Associated Press. "But if it is a political gathering of five people or more, then we will have to take some action."

"If it persists, then we will have to make an arrest," he said.

Social activist Sombat Boonngam-anong, who has helped organise anti-coup protests, posted an explanation of the salute on his Facebook page along with a call to step up the silent acts of defiance.

"Raising three fingers has become a symbol in calling for fundamental political rights," wrote Sombat, a member of the "Red Shirt" protest movement that had backed the now-ousted government and warned it would take action if there was a coup. He called on people to raise "3 fingers, 3 times a day" in public places where there is no police or military presence.

"Let's escalate the anti-coup movement three times a day together," he said, stipulating that protesters should raise their right arm and stay still for 30 seconds.