Thai authorities warned demonstrators against gathering to mark the anniversary of the Siamese Revolution on Wednesday, citing the kingdom's soaring Covid-19 infections.

Key pro-democracy groups have announced plans for demonstrations across Bangkok on Thursday to commemorate the 89th anniversary of the Siamese Revolution, an uprising that transformed Thailand from absolutism to a constitutional monarchy.

Mass protests against the government of Prime Minister Prayut Chan-O-Cha shook Bangkok in the second half of last year, fuelled by discontent at the former military chief who came to power via a 2014 coup, but tailed off as covid cases rose.

Pro-democracy protests rocked Thailand last year, with over 150 people charged under the country's tough royal defamation laws
Pro-democracy protests rocked Thailand last year, with over 150 people charged under the country's tough royal defamation laws AFP / Jack TAYLOR

The kingdom is now in the throes of its third wave of the virus, registering high daily numbers of infections and deaths, which has seen authorities clamp down on gatherings.

"The police is ready to take care of the security for protests tomorrow... Police will focus on maintaining peace and order and follow the disease control regulation," Bangkok metropolitan police commissioner Pakapong Pongpetra said.

"Anyone who violates the laws during protests will have legal actions taken against them," he said, adding authorities "will not use force if it's not necessary".

The protest venues announced by the groups include key city roundabouts and public spaces -- including the Democracy Monument and the downtown skywalk at a popular shopping district.

A rise in Covid-19 cases in 2021 slowed down the protest movement that even called for reforms to the monarchy
A rise in Covid-19 cases in 2021 slowed down the protest movement that even called for reforms to the monarchy AFP / Lillian SUWANRUMPHA

There are also demonstrations planned across the country, from the tourist city of Chiang Mai in the north to the southern province of Nakhon Si Thammarat.

The pro-democracy movement sent shockwaves through Thailand's establishment, particularly their most controversial demand -- a call to reform the kingdom's unassailable monarchy.

Some 150 people have been charged since the movement started, with key leaders hit with multiple counts under Thailand's tough royal defamation laws.

Many of them were released on bail under conditions that include not protesting, but they are expected to be leading Thursday's marches.

"Come out tomorrow to show our fight is not over," tweeted prominent student leader Panusaya Sithijirawattanakul, who was released on bail in May.

"We will continue fighting."

At the peak of the movement last year, the protests drew tens of thousands of demonstrators calling for an overhaul to Prayut's administration, but momentum has slowed in 2021 because of an uptick in Covid cases.

The kingdom has recorded more than 228,500 total cases of Covid and 1,744 deaths.

Coronavirus