CAIRO, Egypt -- The Muslim Brotherhood ramped up its calls Thursday for nationwide protests against Egypt's military-backed government as security forces arrested more two top figures of the organization.

The Brotherhood's call for mass protests and sit-ins Friday will test how much the fierce security crackdown has crippled the group and if they can still mobilize their base in the face of widespread public anger against them. Egypt's security forces have increased their presence in the streets ahead of the planned demonstrations and accused the Islamist group on Thursday of using rallies to create chaos.

It is unclear whether hard-line Islamist parties within the Brotherhood's alliance will take part in the protests. Some have begun distancing themselves from the group.

There are concerns that Friday's protests could tailspin into another bout of violence. Bloodshed peaked two weeks ago when police, backed by snipers and bulldozers, attacked two Brotherhood-led sit-ins in the capital protesting the July 3 military ouster of President Mohammed Morsi, a longtime leader in the group.

The move sparked violence that killed more than 1,000 people, most of them people opposed to Morsi's ouster. Many retaliated by attacking police stations, torching churches and setting government buildings on fire.

The Brotherhood, once Egypt's most powerful political group, has been weakened by mass arrests of hundreds of top and midlevel leaders. Police have also gone after members' relatives.

State television and police officials said Thursday that officers arrested outspoken former Brotherhood lawmaker Mohammed el-Beltagy and former Labor Minister Khaled el-Azhari. It is not clear why el-Azhari was arrested, but el-Beltagy was wanted by police for allegedly inciting violence and alleged involvement in an assault on a police officer kidnapped by protesters at the main Cairo sit-in before it was cleared.

El-Beltagy was among four top Islamists who took to the airwaves in the past 24 hours to try and whip up support for Friday's protests after a week of small and scattered rallies. They called on people to stand their ground against the "failed, bloody military regime."

The Interior Ministry, in charge of police, said in a nationally televised statement Thursday that its forces would deal with "firmness" against acts that threaten national security and that police had orders to use deadly force in defence of public and private property. The ministry said that the Brotherhood's calls are aimed at stirring chaos. A nighttime curfew in Cairo and 13 other provinces will start earlier on Friday at 7 p.m. and end 6 a.m.

The Brotherhood released a four-page Arabic statement Thursday, part of which called on security forces to disobey orders "to kill." The group and their supporters have flashed four fingers and used it as their symbol in online and street campaigns for protests. It comes from the sit-in around Rabaa el-Adawiyah mosque, which in Arabic means fourth.

A photo purporting to show el-Beltagy just after his arrest has him flashing the sign and smiling between two masked policemen.

Former Brotherhood lawmaker Essam el-Erian warned in one of the televised messages against the return of "the police, intelligence and security state" following the country's 2011 uprising that ousted autocratic President Hosni Mubarak. He said the government's so-called war on terror is fabricated and aims at reversing the gains of the revolution when millions fought back against police brutality and other injustices.

The pre-recorded messages all aired on Al-Jazeera Mubasher Misr, an affiliate of the Qatar-based broadcaster. Egypt's interim government on Thursday deemed the local Al-Jazeera affiliate a national threat, moving closer to banning its broadcast. It is among the only channels seen as sympathetic to the Brotherhood still broadcasting after others were ordered closed.