Thousands of protesters marched on the German embassy in Bangkok on Monday to ask for an investigation of the Thai king's activities during stays in Germany, as months of protests have given rise to growing calls for reforms of the monarchy.

Student-led protests that began by demanding the removal of Prime Minister Prayuth Chan-ocha, a former military ruler, have also become the biggest challenge in decades to a monarchy that the Thai constitution says must be revered.

"We have three demands – first that the PM must resign, second that we want a new constitution from the people not the government and third we want to reform the monarchy," said one 16-year-old student protester who declined to be named.

Criticizing the monarchy can mean a 15-year jail sentence in Thailand, but the protests have swept away the longstanding taboo on discussing it. 

Protesters accuse the monarchy of helping to enable decades of domination by military rulers. They also complain about spending on the king's European visits at a time the coronavirus has hit the tourism-reliant economy hard.

Germany has said it would be unacceptable for King Maha Vajiralongkorn to conduct politics in Germany and Foreign Minister Heiko Mass said the European country continued to look into his behavior during long sojourns in Bavaria.

Thailand's biggest opposition party earlier on Monday called for Prime Minister Prayuth Chan-ocha to resign, as parliament opened a special session called by the former junta leader to discuss months of protests.

Demonstrations which initially demanded the departure of Prayuth and a new constitution have increasingly turned their attention to the monarchy, calling for reforms to curb the powers of King Maha Vajiralongkorn.

"The prime minister is a major obstacle and burden to the country. Please resign and everything will end well," said Sompong Amornvivat, leader of the opposition Pheu Thai party, the largest single party in parliament.

Prayuth called the parliamentary session this week after the imposition of October 15 emergency measures to end the demonstrations – including a ban on protests – only inflamed anger and brought tens of thousands onto Bangkok streets.

"I'm confident that today, regardless of our different political views, everyone still loves the country," Prayuth said in his opening address.

(With input from Reuters)