MANAMA (Reuters) - Authorities in Bahrain said on Saturday they would take legal action against the opposition Al Wefaq group after it held a banned anti-government march, a move the group said was the most serious action against it since mass protests last year.
The Interior Ministry said it was acting after the group went ahead with a march on Friday in Manama, the capital, during which six protesters were arrested.
"The Interior Ministry holds Al Wefaq responsible for violating the law and encouraging their supporters to participate in a non-sanctioned event," said a statement from the ministry distributed by the government's public relations office.
The opposition has defied government bans on demonstrations before, but this is the first time the authorities have threatened legal action since a protest movement dominated by majority Shi'ite Muslims erupted last February.
The United States has called on Bahrain, which hosts the U.S. Fifth Fleet, to engage with the opposition and hold more talks with its representatives.
It was not immediately clear what the action might entail, but the move appeared to represent an escalation of government attempts to put pressure on Al Wefaq, a group it has threatened to ban in the past.
Opposition parties led by Al Wefaq are demanding full powers for the elected parliament so that it can legislate and form governments. Many Shi'ites complain of being politically and economically marginalised, a charge the government denies.
Police used teargas and stun grenades to break up Friday's march, which dozens of protesters took part in.
The statement from the Interior Ministry said Al Wefaq had been told a day earlier that the protest had not been authorised and that demonstrators engaged in "the blocking of roads, vandalism and spreading fear and concerns among the business owners in the area".
"The ministry affirms its support for free speech but reminds all citizens that freedom of expression does not include vandalism, spreading fear amongst the community and attempting to create chaos. The ministry has taken legal action to file a case against Al Wefaq," it said.
The ministry had also filed cases with the public prosecutor against the six arrested protesters, it added.
"Many times they go for the illegal march, but this time they targeted the heart of the capital and that's bad for businesses and the economy. That's the problem," said a government adviser who asked to remain anonymous.
He said unauthorised rallies in other parts of the country would not have prompted legal action and that he thought it unlikely any action would include banning Al Wefaq.
Al Wefaq criticised the authorities for banning the march and said in an emailed statement that the riot police had used violence against peaceful protesters.
"The regime arbitrarily blocked all the ways to the capital Manama early in the afternoon dividing the capital into security squares filling them with its forces who used all kinds of weapons against protesters," it said.
Opposition leaders said the tone of the Interior Ministry statement was stronger than in the past.
"Wefaq has been threatened in the past, but the level and the wording, all of these show it is possibly more serious than at any other time," Jasim Husain, one of the group's leaders, said.
"But they are not yet threatening a ban. The wording is legal action."
Armoured vehicles and riot police had closed off some of the main roads leading into the city on Friday, but dozens of protesters attended the march, which had been billed as an event to support "freedom for prisoners of conscience".
Last week, a march attended by tens of thousands of demonstrators that had also been organised by Al Wefaq together with other opposition groups and which the authorities had approved passed off without incident.
On Tuesday, a civilian court upheld jail sentences of between five and 25 years for the leaders of last year's uprising, prompting condemnation from Al Wefaq.
The United States in June said it was "deeply disappointed" that a Bahraini court had upheld verdicts against medics accused of participating in last year's uprising, while President Barack Obama last year called on the government to talk to Al Wefaq.
Writing by Angus McDowall in Riyadh; Editing by Andrew Osborn