Thai protesters splashed paint, daubed graffiti and sprayed water on police headquarters in Bangkok Wednesday, a day after dozens were hurt by tear gas and water cannon in the worst violence in months of anti-government demonstrations.

Since July, youth-led protests have become the biggest challenge to the establishment in years with their demands for the ousting of Prime Minister Prayuth Chan-ocha, a new constitution and reforms to curb the monarchy's powers.

Tens of thousands joined Wednesday's protest, which turned the outside of the Royal Thai Police headquarters into a multi-coloured mess. Protesters placed dog food at the gate for police as an insult to what they called "slaves of tyranny."

Some sprayed anti-monarchy graffiti.

Protesters said they were enraged by the police response to Tuesday's demonstration at parliament and the rejection by lawmakers of a constitutional reform proposal that could have considered the role of King Maha Vajiralongkorn.

"We came here solely because of our anger," said Panusaya "Rung" Sithijirawattanakul, one of the protest leaders.

Police barricaded themselves inside and did not intervene.

Dozens hurt in previous day's rally

In case the police used water cannon, protesters brought inflatable pool ducks to use as shields. The ducks have also become protest mascots.

According to police spokesperson Kissana Phathanacharoen the protesters vandalized public property and threw projectiles that could have hurt officers. As a result, he said charges will be brought against them.

Large inflatable ducks that have become protest mascots are passed around the crowd at the pro-democracy rally in Bangkok Wednesday. (Sakchai Lalit/The Associated Press)

Protests since July have been largely peaceful.

But Bangkok's Erawan Medical Centre said at least 55 people were hurt on Tuesday, with 32 suffering from tear gas and six with gunshot wounds. Protesters also scuffled with royalist counter-protesters.

Police said a royalist had been charged with possession of a weapon and ammunition after the protest, but did not accuse him of opening fire.

Royalists say they believe that calls for reforming the monarchy are a way to abolish it. Protesters deny that.

Proposal allowing for monarchy reform rejected

In parliament, where lawmakers voted on seven options for ways to change the constitution, a proposal that would have opened the way for discussion of the king's role failed to win enough votes for approval.

Two proposals were adopted that would allow for discussion of constitutional change without affecting the monarchy.

"We will not touch on that," said Wirat Ratanaset of the ruling coalition.

Police stand behind coils of barbed wire at their headquarters during the rally in Bangkok on Wednesday. (Wason Wanichakorn/The Associated Press)

The vote was not a surprise. Prayuth's supporters have a majority in parliament, where the entire upper house Senate was appointed by the junta he led after a 2014 coup until a disputed election last year.

The Royal Palace has made no comment since the protests began.

Protesters said their next event would be at the Crown Property Bureau on Nov. 25 over the management of the palace fortune, which the king has taken into his personal control. The fund is valued in the tens of billions of dollars.

Protesters said there would be seven more days of demonstrations after that.