By Patpicha Tanakasempipat and Matthew Tostevin
BANGKOK (Reuters) - Describing themselves as "human beings, not dust", thousands of Thai protesters demanding reforms of the monarchy marched to the German embassy in Bangkok on Monday to put pressure on King Maha Vajiralongkorn, who spends much of his time in Germany.
Germany has said it would be unacceptable for King Maha Vajiralongkorn, 68, to conduct politics there and Foreign Minister Heiko Maas said the European power continued to look into his behaviour during sojourns in Bavaria.
Months of student-led protests that began by calling for the removal of Prime Minister Prayuth Chan-ocha, 66, a former military ruler, have become the biggest challenge in decades to a monarchy that the Thai constitution says must be revered.
The Palace has made no comment since the start of the protests. A government spokesman did not immediately respond to a request for comment on Monday's protest.
At the German embassy, a statement was read from "the People" accusing the king of interfering in Thai politics. It said that having failed to get "the royal puppet" to listen - a reference to Prayuth - they had come to "the puppet owner".
"The era of change has arrived. The flowing stream of democracy cannot be stopped," it said. "Down with feudalism, Long live the people."
A separate letter asked Germany to investigate:
* Whether the king conducted state business from there.
* Whether he must pay inheritance tax there after the death of his father King Bhumibol Adulyadej in 2016.
* Accusations of human rights abuses.
* The king's lifestyle in Germany.
It was signed: "Best regards from fellow human beings, not dust" - referring to a Thai expression that people are but dust under the king's feet.
Criticising the monarchy can mean 15 years in jail, but the protests have swept away the taboo on discussing it.
Protesters accuse the monarchy of helping enable decades military domination. They also complain about royal spending in Europe when the coronavirus has hit the tourism-reliant economy hard.
Thailand's political crisis has made the king's presence a challenge for Germany. "We are monitoring this long-term," Maas said. "It will have immediate consequences if there are things that we assess to be illegal."
The king is currently in Thailand.
In a special session of the Thai parliament on the crisis, Prayuth's opponents urged him to stop using the monarchy to justify his hold on power and resign.
The opposition Move Forward party referred to an incident around a royal motorcade that was used to justify tough emergency measures on Oct. 15. The measures, now lifted, backfired when tens of thousands of people took to the streets.
Prayuth rejected protesters' demand to resign and said last week it was his government's job to protect the monarchy.
"I’m confident that today, regardless of our different political views, everyone still loves the country," he told parliament.
Prayuth took power in a 2014 coup and protesters say he engineered last year's election to keep control. He says the ballot was fair.
Protesters want the king to be bound by the constitution and to reverse changes giving him personal control of the palace fortune and some army units.
They said the Germany embassy had listened.
"Since we cannot ask for this clarity from our government, we seriously hope the German government can do it for us," protest leader Patsaravalee 'Mind' Tanakitvibulpon, 25, told the crowd.
She was among scores of protesters arrested in a crackdown under emergency measures. She was freed on bail. Several prominent leaders are still detained.
Before protesters reached the embassy, a couple of dozen royalists assembled there, shouting: "Long live the king. We will protect him with our lives."
(Additional reporting by Juarawee Kittisilpa, Orathai Sriring, Satawasin Staporncharnchai in Bangkok, Sabine Siebold in Berlin; Writing by Matthew Tostevin; Editing by Clarence Fernandez and Mark Heinrich)