By Kaweewit Kaewjinda and Panarat Thepgumpanat

BANGKOK, Nov 24 (Reuters) - Seven Thai policemen have been charged for offences ranging from accepting bribes to insulting the monarchy as a probe that has led to the arrest of two senior officers widens, a police spokesman said on Monday.

Thailand's lese-majeste law is the world's harshest and makes it a crime to defame, insult or threaten the king, queen, heir to the throne or regent. It is rare for high-ranking officials to face charges of lese majeste.

Those who are convicted face up to 15 years in prison.

Police Lieutenant General Pongpat Chayaphan, commissioner of the Central Investigation Bureau, and his deputy Police Major General Kowit Wongrungroj were accused of using the royal establishment for personal benefit, said police spokesman Lieutenant General Prawut Thavornsiri.

Prayuth Chan-ocha, the former army chief who led a coup in May and was appointed prime minister in August, is a self-proclaimed royalist and has vowed to root out critics of the monarchy.

There has been an unprecedented spate of lese-majeste cases since the military seized power.

But it was rare for civil servants, much less senior police officers, to be charged with the crime, said David Streckfuss, a Thailand-based scholar who monitors such cases.

"In the political turmoil of this past decade, lese-majeste has been used almost exclusively to silence the threat posed by the red-shirt movement to the Bangkok establishment," said Streckfuss, referring to supporters of former Prime Minister Yingluck Shinawatra and her brother, Thaksin.

The Thai army seized power after months of protests aimed at ousting Yingluck, saying it needed to restore order after nearly 30 people were killed in sporadic political violence.

The military government has vowed to reform the police, widely regarded as one of Thailand's most corrupt institutions.

The junta has already systematically purged the police force of many officials deemed loyal to Thaksin, who was ousted by the army in another coup in 2006.

A total of twelve people have been charged in relation to the case that involves the seven police officers, Prawut told Reuters.

A search of Pongpat and Kowit's homes revealed assets worth a total of more than a billion baht ($30 million), he said.

Pongpat was head of the Crime Suppression Bureau, a division of the CIB, during the arrest of notorious Russian arms dealer Viktor Bout in a joint sting with the FBI in 2008. (Writing by Andrew Marshall, Editing by Simon Webb and Clarence Fernandez)