Security forces are on high alert across Libya as the country marks two years since the start of the revolt that toppled Muammar Gaddafi after four decades of authoritarian rule.

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Borders have been closed and some international flights suspended amid fears of a new outbreak of violence.

The anniversary of the uprising that ended with Gaddafi’s killing in October 2011 comes as Libya's new rulers battle critics calling for a "new revolution" and accusing them of failing to usher in much-needed reforms.

It also comes a day after security officials said they had arrested four foreigners in Benghazi on suspicion of being Christian missionaries and printing books about Christianity.

On Friday, thousands of people gathered in the main cities of Tripoli and Benghazi to celebrate the initial February 15, 2011 protest that ignited the revolt two days later.

There is no official programme for Sunday's anniversary, but the authorities have taken steps aimed at preventing any violence on a day when spontaneous celebrations are expected.

Libya's borders with Egypt and Tunisia were closed from Thursday for four days, and all international flights have been suspended except at the airports of Tripoli and second city Benghazi - the cradle of the "February 17 revolution".

Checkpoints across Tripoli

Ali Zeidan, the country’s prime minister, said the measures were taken to avoid "any bid to undermine Libya's security and disrupt celebrations marking the anniversary of the revolution".

Checkpoints have also been set up across Tripoli and in Benghazi.

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Opposition groups are demanding that former Gaddafi regime officials be barred from holding public office, and a leaflet circulated in Tripoli calls for a "popular revolt" and civil disobedience to bring down the current regime.

It is unclear who is behind the leaflet and the calls for protests, but Libyan officials and several organisations, including Islamic groups, accuse remnants of the former regime of fomenting protests to "sow disorder and instability".

The authorities have said special permits will be needed for "peaceful protests", and have threatened force against anyone who tries to derail anniversary festivities.

Residents of Benghazi - which has been hit by violence targeting international agencies and diplomatic missions - have set up neighbourhood watches.

The city was rocked on September 11 last year by an assault on the US consulate there that killed US ambassador Chris Stevens and three other Americans.

Despite Libya holding its first free elections in July last year, analysts say the country is not yet politically mature after four decades of dictatorship under Gaddafi.

Source: Al Jazeera and agencies