Thousands of people rallied on Saturday in central Bangkok for the biggest protest since before the 2014 coup, as Future Forward Party leader Thanathorn Juangroongruangkit declared it was “just the beginning” of a movement against anti-democratic forces.

Mr Thanathorn called the protest on Friday after moves to ban his party, which has emerged as the most vocal challenger to the government of former military ruler Prayut Chan-o-cha.

“Today is a show of strength so that in future others may join us,” Mr Thanathorn told the crowd on the skywalk linking the National Stadium and Siam BTS stations. “We’re just here today as a test run. Prayut, don’t be afraid yet. The real thing is next month.”

Several protesters chanted: “Prayut out!” while many held placards showing how they felt. The one-hour rally ended shortly after 6pm and the party thanked supporters on Facebook for their “powerful expression”.

“We’ll meet again in the future,” read the post, adding “the same roll of film is being rerun but the end won’t be the same. … Today is just a taste”. 

Mr Thanathorn went live on Facebook on Friday calling for a peaceful protest in the face of a possible ban on his party.

He urged people to stage a flash mob under the theme “When the voice we chose for Parliament is worthless, it’s time for people to make their voice heard.”

Police in Pathumwan district said they had not received a request for a gathering in line with the law on public gatherings, but they made no apparent attempt to block Saturday’s rally.

Prime Minister Prayut complained earlier that it was inappropriate to organise a demonstration towards the end of the year when people were in a celebratory mood.