The rallies were called on Saturday evening after Prime Minister Prayuth Chan-ocha ignored the protesters’ deadline to withdraw.
The main demands of the protesters include a more democratic constitution and reforms of the monarchy.
Public criticism of the monarchy is unprecedented in a country where the royal institution has been considered sacrosanct.
Protesters accuse Prayuth, who led a 2014 coup as army chief, was unfairly returned to power in last year’s general election because laws were changed to favor a party pro-military.
Protesters also say the constitution, drafted and enacted under military rule, is undemocratic.
Prayuth’s government has convened parliament in a session – which is slated to begin Monday and last for two days – in an attempt to defuse weeks of almost daily protests.
“The only way to achieve a lasting solution … is to discuss and resolve these differences through the parliamentary process,” he said last week.
Prayuth also lifted the state of emergency he imposed a week earlier on Thursday that made protest rallies illegal.
Protesters were unimpressed by his efforts to appease them, saying they were insincere.
Several noted on social media that the talking points put forward by the government for debate were not intended to address the concerns of protesters, but were thinly disguised criticisms of the protests themselves.
Tony Cheng, from Al Jazeera, who reports from Bangkok, said the protesters were speaking out fully and reiterating their demands.
“There are thousands of demonstrators in the streets. The atmosphere is more relaxed than what we have seen in recent weeks. There are police officers here, but they step back and direct traffic where they can. The authorities did not shut down the transportation system, which they have already done, ”Cheng said.
The protest organizers called for a Monday afternoon march that will take them to the German embassy in central Bangkok, away from the parliamentary complex on the outskirts of the city.
The march apparently aims to draw attention to protesters’ claim that King Maha Vajiralongkorn is spending too much time in Germany.