Tens of thousands of anti-government protesters rallied in Pakistan's capital Saturday, vowing to stay until Prime Minister Nawaz Sharif resigns and new elections are held.
The protesters are part of separate demonstrations led by opposition leader Imran Khan and Muslim cleric Tahir ul-Qadri.
Qadri told his supporters Sharif should be arrested when he steps down.
Khan reiterated his allegation that last year's election that brought Sharif's political party to power was marred by "massive rigging.". He also repeated his demands for new elections under a reformed electoral commission.
Authorities have tightened security in key areas of Islamabad to prevent violence during the protests.
The protests constitute the biggest challenge yet to Sharif's year-old government and raise concerns about political instability in the nuclear-armed nation.
Khan and Qadri say the government is corrupt. Their two movements are not officially allied.
Sharif has not indicated he intends to resign, and he denies the election-rigging charges.
Earlier this week, Sharif called for a discussion of the issues, rather than political action. He has warned against what he calls "any effort to create anarchy and play with the constitution," adding the government "will not allow anyone to paralyze the state machinery or incite bloody riots."
Qadri is demanding justice after a police crackdown on an anti-government rally in June, in which about a dozen people died. He says authorities are targeting his supporters using anti-terrorism laws.
On Friday, clashes broke out in the city of Gujranwala after gunshots were fired at Khan's vehicle as he led an anti-government march to Islamabad. The shots hit Khan's vehicle, but his spokeswoman said he was not hurt. A stone-throwing mob also attacked Khan's convoy.
Around 25,000 security forces are guarding the capital's streets. Internet and mobile services have been suspended in many areas.
Thousands Head to Opposition Rally in Pakistan
Supporters of Pakistan's cricketer-turned-politician Imran Khan, in green shirts, clash with supporters of the ruling party in Gujranwala, Pakistan, Aug. 15, 2014.
A supporter of Pakistan's ruling party throws a rock at the procession of cricketer-turned-politician Imran Khan in Gujranwala, Pakistan, Aug. 15, 2014.
Clashes broke out between supporters and non-supporters of politician Imran Khan, with both parties throwing rocks at each other, at the Freedom March, in Gujranwala, Aug. 15, 2014.
Policemen move to disperse residents after they threw stones at participants of the Freedom March, led by cricketer-turned-opposition politician Imran Khan, in Gujranwala, Aug. 15, 2014.
Supporters of cricketer-turned-opposition politician Imran Khan take part in the Freedom March in Gujranwala, Pakistan, Aug. 15, 2014.
Some information for this report comes from AP and AFP.