Insulting the monarchy is a crime, punishable by up to 15 years in prison.

A Thai-language hashtag that translates to #whydoweneedaking? was one of the top trending topics on Twitter in Thailand after an overseas Thai activist posted about King Maha Vajiralongkorn continuing to travel in Germany during the coronavirus crisis.

King Vajiralongkorn, 67, who was crowned last year, has a second home in Germany. He spends much of his time outside Thailand.

The number of confirmed cases of coronavirus in Thailand has risen 14-fold during March to 599 cases, the second highest-number in Southeast Asia after Malaysia, according to official data announced by national health authorities. One person has died.

The hashtag questioning the monarchy was used more than 1.2 million times in 24 hours by Sunday, according to data on Twitter based on hashtags trending for users in Thailand.

Thailand's Royal Palace did not respond to requests for comment on the posts.

On Twitter, Minister of Digital Economy and Society Puttipong Punnakanta posted a warning to citizens against breaking laws on online content, accompanied by an image of a handcuffed hand above a keyboard.

"I'd rather not comment," he told Reuters when asked whether his March 22 post was related to those about the monarchy.

"I didn't specify what this was about - this is a general reminder. We follow all issues, like fake news," he said. "We are monitoring regularly as much as we can. We respect self-expression but if it causes damage, we will exercise the law."

When asked whether the government would take action over the posts, government spokeswoman Narumon Pinyosinwat said the situation was being monitored, but any government action would depend on consultations with security agencies.

Among the first to use the hashtag was prominent Thai exile Somsak Jeamteerasakul, who posted on Saturday that the king was travelling in Germany while Thailand was dealing with the coronavirus outbreak. Reuters was unable to verify where the king had travelled during this period.

Thailand was the first country outside China to record a case of the virus in January, but reported only 42 infections before the start of March, according to statements from the Ministry of Public Health.

As cases have surged, more stringent control measures have been imposed. Those include requirements that anyone travelling from outside the country, including Thai citizens, needs special travel papers to fly to the country.

The tourist industry that accounted for more than a tenth of gross domestic product has suffered heavily.

Provincial authorities announced on Saturday that the capital Bangkok will close malls for 22 days although supermarkets will be allowed to remain open. Bars in the city will remain closed for that period, as will schools. Other Thai provinces are increasingly applying restrictions.

Thailand has been a constitutional monarchy since a 1932 revolution ended absolute royal rule, but the monarchy remains a central part of traditional Thai culture. Some Thais consider the king to be semi-divine.

Open disparagement of the monarchy has been rare, but in the past year other Twitter criticism has included a #royalmotorcade hashtag about traffic jams caused by members of the royal family.

The king later ordered police to limit roadblocks for motorcades.