DUBAI (REUTERS) - Islamists charged with a coup plot in the United Arab Emirates (UAE) have said they were abused in custody and, in a statement on social media, demanded an independent inquiry.

The trial of 94 Emiratis, which began in March, is seen as an attempt by the US-allied Gulf Arab state to address what it says is a security threat from the banned Muslim Brotherhood.

The 94 were among dozens of people detained in a crackdown on Islamists over the past year amid heightened worries among officials about a spillover of unrest in other Arab countries.

In an open letter to the UAE president and vice-president published on a Twitter account belonging to the Islamist group Al-Islah, the defendants said they had been insulted, threatened and in some cases subjected to physical abuse after arrest.

It was not clear how many of the 94, most of whom are in custody, had signed the letter or how it had been communicated outside prison.

Most of the detainees belong to Al-Islah, which denies government charges that it is an arm of the Muslim Brotherhood.

Attorney-general Saleem Saeed Kubaish said in January that group members had sought to penetrate state institutions, including schools, universities and ministries, according to state news agency WAM.

The defendants are accused of "belonging to an illegal, secret organisation... that aims to counter the foundations of this state in order to seize power and of contacting foreign entities and groups to implement this plan," WAM said.

Local media, which have attended the trial proceedings, have said the accused deny the charges. Foreign media have been barred from attending.

The group, which signed the letter dated May 7 as 'Prisoners from the Islah group', said they were "unjustly accused of planning to seize rule".

"We ... have been subjected to solitary confinement for many months in tight cells with no windows and with continuous light morning and night," the letter said.

The group complained of insults and threats and said some had been physically abused or had not been allowed to speak to their lawyers.

"(We call for) ... the formation of an independent committee to investigate these abuses," said the group.

Dr Anwar Gargash, the UAE's state minister for foreign affairs, dismissed the letter as "nothing new".

"Actually it is a repetition of the campaign that the Brotherhood has chosen against the nation," Dr Gargash, who is also state minister for Federal National Council Affairs, said on his Twitter page late on Thursday.

"The statement repeats this tale about torture, which it wants to attach to the nation ... and the statement disregards that the Emirates' measures took place within a constitutional and legal framework," he said.