* Head of Bahrain Centre for Human Rights arrested
* Government says Rajab helped provoke rioting, arson
* Hardliners seen taking tough line after Grand Prix (Adds government statement with charges)
By Andrew Hammond and Rania El Gamal
DUBAI, May 6 Bahrain has arrested a prominent human rights activist and critic of the country's ruling family, the Interior Ministry and an activist said on Sunday, as the authorities escalated a crackdown on pro-democracy protesters.
Bahrain, which is ruled by a Sunni Muslim monarchy and hosts the U.S. Fifth Fleet, has been in turmoil since activists mainly from the majority Shi'ite community began protests in February 2011 after successful revolts in Egypt and Tunisia.
Police arrested Nabeel Rajab, head of the Bahrain Centre for Human Rights (BCHR), on return from Beirut on Saturday evening, Sayed Yousif Almuhafda said by telephone from Manama.
Prosecutors then questioned him extensively about his tweets on social media platform Twitter before he was taken to court on Sunday morning on previous charges of organising a protest inside Manama in March, a member of the BCHR told Reuters.
"The police arrested him near the plane's door. They said they had an arrest warrant from the public prosecutions office... At the prosecutor's office, it was all about tweets," he said, adding it was not clear if the questioning about messages on Twitter would lead to new charges.
Rajab shot to prominence last year when he became a trenchant campaigner against the crackdown. With 140,000 followers on Twitter he is one of the most well-known online activists in the Arab world.
Several hundred gathered outside Rajab's home in Bani Jamra west of Manama on Sunday evening, chanting "down with (king) Hamad" and slogans demanding his release.
Rajab faced charges of "inciting illegal rallies and marches online by using social networking websites" and posting "defamatory and humiliating depictions of the public security forces", an Information Affairs Authority statement said.
The statement said his actions had provoked rioting but did not say if that accusation would feature in official charges against him.
"A police investigation also revealed that the defendant's cyber incitement proved detrimental to public security as it fuelled rioting, road blocking, arson, acts of sabotage targeting public and private properties, and the use of petrol bombs and incendiary devices," it said.
"Evidence has been compiled on the defendant's role in instigating online acts such as targeting policemen while on duty that has resulted in serious injuries."
It said Rajab refused to answer questions at the arraignment because he did not recognise the court.
Authorities are also holding protest leader Zainab al-Khawaja after she demonstrated alone on a major highway in April. Prosecutors say she insulted women police officers.
Khawaja became a symbol for protesters after she was dragged from a traffic roundabout in December by women riot police.
Both Rajab and Khawaja, daughter of jailed uprising leader Abdulhadi al-Khawaja who is on a hunger strike, have been detained briefly on several occasions in the past year but this is the first time they were held with intent to press charges.
Justin Gengler, a Qatar-based researcher on Bahrain, said arresting Rajab was an escalation that would please Sunni hardliners who have harangued the government for not crushing protests they view as a Shi'ite attempt to destabilise the country.
"After mobilising Sunnis, the state can only appease them by caving in to their demands for a harsher response to protesters and activists," he said.
The International Federation for Human Rights, in which Rajab is deputy secretary general, condemned the arrest.
"The federation demands the immediate and unconditional release of Rajab and other rights defenders, while it appears that these judicial harassments aim to place blocks against human rights activities," the Paris-based group said.
Rajab and Khawaja have been a thorn in the government's side, organising peaceful protests inside Manama without licences - in contrast to the leading opposition party Wefaq which obtains Interior Ministry approval.
The marches in Manama have sometimes ended violently when police fire tear gas and stun grenades to disperse the protesters and youths throw back petrol bombs.
Their acts of civil disobedience have made them heroes to many Bahraini opposition activists. Western activists, who were eventually deported, joined Rajab for protests in February that marked one year since the protest movement began.
Tensions have risen again since April when Bahrain's Formula One Grand Prix became a lightning rod for protesters and visiting journalists turned their attention to an uprising that has not gone away.
Analysts predicted that hardliners within the ruling family would show their teeth after the Grand Prix, when Bahrain stopped some journalists entering and deported a team from Britain's Channel Four for entering on tourist visas.
A statement on the state news agency warned clerics against incitement to violence, sectarianism, harming the economy, and insulting the judiciary and constitutional institutions - comments apparently directed against leading Shi'ite cleric Sheikh Isa Qassim who led a mass protest in March.
"The cabinet instructed ministries to take legal measures if these violations continue, affirming its total rejection of any bargaining over the nation's security and unity," BNA said.
King Hamad enacted constitutional reforms last week that would boost the elected parliament's powers of scrutiny over ministers and budgets. But the government has not budged on the key demand for a single chamber of parliament with full powers to legislate and form governments. (Editing by Alison Williams)