Bahrain Mirror - AFP: Bahraini human rights activist Nabeel Rajab, a vocal critic of the Gulf island's monarchy, was taken to hospital Tuesday with heart problems, his Twitter account said.

The activist, 51, has been repeatedly detained for organising protests and publishing tweets deemed insulting to the Gulf kingdom's authorities.

He was released last year after King Hamad issued a royal pardon "for health reasons", but was rearrested two weeks ago.

"#NabeelRajab transferred by ambulance to Coronary Care Unit after suffering from heart problems in solitary confinement," his Twitter account said on Tuesday.

Rajab was apprehended earlier this month at his home in the mainly Shiite village of Bani Jamra, near the capital Manama.

His lawyer Jalila al-Sayed said on Tuesday that Rajab will face trial on July 12 on charges "probably related to tweets" which he is said to have either posted or retweeted.

The defence team has not seen the list of charges yet, she told AFP, adding that the tweets were allegedly made in March 2015 and were about unrest at Jaw prison and the war in Yemen.

The head of the Bahrain Centre for Human Rights, Rajab had previously served two years in jail for taking part in unauthorised protests.

Human rights groups condemned Rajab's detention earlier this month.

It "appears to be another alarming example of Bahrain's zero-tolerance stance toward peaceful dissent and activism, which it enforces through arbitrary measures including revolving-doors detention," Amnesty International's James Lynch said.

Home to the US Navy's Fifth Fleet, Bahrain has been rocked by unrest since security forces crushed Shiite-led protests in 2011 demanding a constitutional monarchy and an elected prime minister.

Bahraini authorities have escalated their crackdown on activists in recent weeks.

In May a court more than doubled a jail sentence passed against opposition leader Sheikh Ali Salman.

Authorities also prevented activists from travelling to Geneva to attend a UN Human Rights Council session in June, Amnesty said.

The US State Department said it had complained directly to Manama following Rajab's re-arrest.

His detention came a week after another leading opposition activist, Zeinab al-Khawaja, left Bahrain following her release from jail on "humanitarian grounds".