Hundreds of Thai protesters broke through a police human shield to storm a 'sacred' royal field.
The crowd marched from the Thammasat University campus while being joined by supporters as they headed to the Sanam Luang field in Bangkok.
Sanam Luang is used for Royal events and was closed to the public in 2012.
Police warned demonstrators not to encroach on the land, which has historically been used as the royal cremation ground.
However, the furious mob said they would 'reclaim' the land, which is in front of the city's historic Grand Palace used by the monarchy.
Footage shows the chaotic scenes as crowds pushed through a human barricade formed by police officers from the 'crowd control unit'.
The demonstrators said they were 'occupying' the land which 'belongs to the people'.
The rallies started earlier in the morning at the city's Thammasat University, where activists and students broke a locked gate to enter.
Defying heavy rain and police warnings that the gathering was illegal, they then listened to a series of speakers and chanted.
Prime Minister Prayut Chan-o-cha that the event could risk causing a spike in Covid-19 cases but a hardcore number of demonstrators still arrived.
He said: "When you gather in mobs, you are creating an enormous risk of new infections.
And with that, you also create enormous risk to the livelihoods of tens of millions of fellow Thai citizens.'' Police chiefs warned that the gathering was illegal as it had not sought permission but said those attending would not be arrested.
However, the activists defied the warnings.
Dozens of rallies have been held in recent months amid anger from activists.
Opponents of the ruling military party, which seized power in a coup in 2014 and won elections in 2019, are critical of the army's strong-arm tactics to silence critics to maintain its grip on power.
Wide-ranging complaints include a lack of transparency, low levels of democracy, economic equality and issues with the country's reputation for police and political country.