Deputy Prime Minister Prawit Wongsuwan told reporters authorities have "solid evidence" that five people arrested planned to carry out bombings in the capital and nearby provinces over five days late in October.

"We would not have taken action [against these suspects] if we didn't have solid evidence," he said, adding the public should not panic as security forces and intelligence agencies were doing their jobs.

All of the five are from Thailand's southernmost Malay-Muslim provinces, where almost 7000 people have been killed as Malay insurgents have waged a separatist war since 2004.

In August a series of co-ordinated bombings and fires hit seven provinces, including Phuket, the most popular Thai destination for Australian tourists.

Confirmation of the plot comes as Thailand's military government urges calm and moves to reassure citizens about the royal succession following the death of 88-year-old Bhumibol, who ruled for 70 years and was a unifying figure in the country that has seen a decade of political upheaval.

Prime Minister Prayuth Chan-ocha said Crown Prince Maha Vajiralongkorn, the king's 64-year-old only son, could ascend to the throne sometime after an initial 15-day mourning period.

He also confirmed the government's intention to allow elections to be held late in 2017.

"Nothing has changed. The policies of this government, the laws, the constitution, and the elections will stick to the roadmap," he said.

Vajiralongkorn reportedly asked not to become king immediately as he wanted to mourn the death of his father with the people.

The decision came as a surprise, as the king designated Vajiralongkorn heir in 1972 and in the past heirs have immediately ascended to the throne after being invited to do so by parliament.

Mr Prayuth said last week that the coronation will not take place until after the king's cremation, which will come after a one-year official mourning period.

But under Thailand's constitution and royal procedure, the heir can ascend to the throne before a coronation is held.

The last time Thailand was without a king was in 1932, when King Prajadhipok​ abdicated and it took five days to name King Ananda Mahidol.

Vajiralongkorn has made no public comment since his father's death at a Bangkok hospital on October 13 following a long illness.

Millions of Thais, mostly wearing black, have been mourning the king's death with dignity,  but there been several reported incidents where mobs have attacked those seen as disrespecting the monarchy.

A video clip of a woman being slapped in Bangkok for allegedly insulting the monarchy went viral on social media. The woman is believed to be suffering a mental illness.

Another attack was live-streamed on Facebook.

The government is also concerned about Thais abroad posting insulting comments about the monarchy.

Justice Minister Paiboon Kumchaya told reporters that while agencies in Thailand are monitoring the comments they will not be able to act against people outside Thai jurisdiction.

But he urged Thais in the country not to share any offending material originating abroad, saying that act would be considered illegal and could carry stiff prison sentences.

Threats have been made against the BBC's Bangkok-based reporter Jonathon Head, who has been reporting the king's passing.

Piyapong Klinpan, a spokesman for the National Council for Peace and Order, which represents the military that seized power in a 2014 coup, urged the public not to use violence and to allow authorities to handle cases based on the law.

The ABC's international network Australia Plus remains blocked in Thailand, with its local cable provider switching the service to continuous pictures and videos of the king.

Concerts, festivals and all celebrations have been cancelled across the predominantly Buddhist country of 64 million people.