Tens of thousands of black-clad Thais gathered outside Bangkok's Grand Palace on Saturday to sing the royal anthem in a striking display of devotion to the recently deceased King Bhumibol Adulyadej.

The monarch, who died on October 13 following years of poor health, was seen as a guiding light and rare source of unity in a kingdom dogged by corruption and political rivalries.

His death has plunged the nation into grief, with the government declaring a one-year mourning period and urging the public to don black and dial down all festivities for at least 30 days.

Crowds have been massing outside his glittering Bangkok palace for the past week, with many journeying from far away provinces to pay respects to a man celebrated as the father of the nation.

Some have pitched tents on a large grassy field outside the royal compound, while others have been sleeping under the stars on simple bamboo mats.

On Saturday, a sea of black filled the field and surrounding roads to stand and sing a royal anthem alongside a 100-piece orchestra and professional choir.

Many held up portraits or bank notes bearing the face of the bespectacled king as they sang in unison, some through tears.

‘I came here to sing a song and pay my last respects to his majesty the late king,’ said Chotika Pattanateeradej, who like most was dressed all in black despite Bangkok's sweltering midday sun.

‘Many people have come today and they are helping each other. I feel very proud,’ she added.

Public displays of mass devotion have been encouraged by Thailand's arch-royalist military rulers, who grabbed power in a 2014 coup many believe was staged to ensure a smooth succession.

A flurry of free or discounted bus, train and plane rides have helped move thousands to the capital, where city workers and volunteers are providing free food and medical care to mourners.

While the outpouring of grief has been overwhelmingly sober and dignified, it has also unleashed small pockets of ultra-monarchist forces that have shamed, mobbed and in some cases beaten Thais seen as criticising the monarchy.

The government has condemned this vigilantism but stepped up its own surveillance of royal defamation -- a crime that carries 15 years in jail for each offence under the kingdom's lese majeste law.

The legislation has seen scores of Thais thrown behind bars -- sometimes for decades -- for perceived slights against the monarchy.

All media based inside the country must heavily self-censor to avoid violating the law, which has been wielded with renewed vigour by the junta.