BANGKOK (AFP) - An "unprecedented" number of people have been charged with insulting Thailand's monarchy since the coup, Amnesty International said on Thursday, with 14 Thais indicted under the controversial lese majeste law in less than four months.

Revered King Bhumibol Adulyadej, 86, is protected by one of the world's toughest royal defamation laws - anyone convicted of insulting the king, queen, heir or regent faces up to 15 years in prison on each count.

"An unprecedented number of people have been charged with lese majeste offences... with fourteen charges or prosecutions initiated since the coup," Amnesty said in a report about rights since the military takeover on 22 May.

The rights group said commentators calling for reform of the law or those previously jailed for royal defamation "appear to have been targeted" in the lists of people the junta required to report to them after seizing power.

Last month, two activists were charged with breaching the law during a university play, which featured a fictitious monarchy, staged in October 2013 on the anniversary of a student-led uprising.

In another recent case, a 28-year-old musician was sentenced to 15 years in jail for writing insulting Facebook posts about the monarchy between 2010 and 2011.

Junta chief and recently appointed prime minister, Prayuth Chan-O-Cha, has said he was forced to take power after months of protests against Yingluck Shinawatra's administration left 28 people dead and hundreds wounded, effectively paralysing her government.

Far-reaching violations

In the days following the coup, more than 570 people were officially ordered to report to authorities, according to Amnesty, which estimates the number would be higher if informal orders were recorded.

"Three months since the coup, a picture emerges from our investigations of widespread and far-reaching human rights violations perpetrated by the military government that are ongoing," said Richard Bennett, Amnesty International's Asia-Pacific Director.

In his weekly televised speeches, Prayuth has reiterated the importance of the royal defamation legislation.

"The laws are aimed at protecting the royal institution. Thailand's strength is based in the monarchy institution and stability," he said last Friday.

Some experts believe that a struggle is unfolding to decide who will run Thailand's government when the more than six-decade reign of the ailing king eventually ends.

The succession is a taboo topic in Thailand and its discussion is restricted under the royal defamation law. But critics says that law has been politicised, noting that many of those charged in recent years were linked to the "Red Shirts" protest movement, which is broadly supportive of fugitive former premier Thaksin Shinawatra.

Thaksin clashed with the royalist establishment before his overthrow in a coup in 2006, and shortly before May's army takeover his younger sister Yingluck was ousted as prime minister in a controversial court decision.

Rights activists have also voiced concern over the fate of suspects under martial law, imposed by the army two days before the coup, as this means they could face military courts with no right of appeal. He has ruled out holding new elections before October 2015, despite international appeals for a return to democracy.