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ISLAMABAD, Pakistan -- Anti-government protesters armed with slingshots and hammers clashed repeatedly with police Sunday in the Pakistani capital, and the powerful military cautioned the prime minister against further use of force in the crisis that has triggered the biggest challenge yet to his authority.

At least three people were killed and nearly 400 admitted to hospitals, officials said, in clashes that started Saturday night and continued to boil over Sunday.

Prime Minister Nawaz Sharif met with top advisers seeking a way to ease the violence, which has raised the stakes in the two-week sit-in led by opposition politician Imran Khan and fiery cleric Tahir-ul-Qadri. They want the prime minister to step down over their allegations of massive voting fraud in the election that brought him into office last year in the country's first democratic transfer of power.

Sharif's party was elected in a landslide in which observers found no evidence of widespread fraud. The prime minister has refused to step down, and negotiators have tried to convince Qadri and Khan to end their protests.

The military weighed in after a late Sunday meeting, saying that it had "serious concern" about the crisis and the "violent turn it has taken."

"Further use of force will only aggravate the problem," military leaders said in a statement that called on political figures to resolve their differences swiftly.

The demonstrations were mostly peaceful until late Saturday, when protesters headed toward the prime minister's residence. When the crowd started removing shipping containers used as barricades, police fired salvos of tear gas that forced protesters back.

Authorities have said they had no choice but to use force on the demonstrators, some of whom wore gas masks.

Hundreds of people were arrested, said Islamabad Police Chief Khalid Khattak. There were scattered clashes Sunday morning, and protesters appeared to regroup later in the afternoon.

By evening, large groups of protesters were located in pockets around the parliament and some had spread into other parts of the city, said police official Amir Paracha. He said they had taken shields from police and were wielding iron rods, batons, stones and bricks.

The injured included women, children, journalists and police officers who had been hurt by tear gas shells, batons and rubber bullets, said Dr. Javed Akram, who heads the capital's main hospital.

Railways Minister Saad Rafiq said the government would investigate the conduct of officers who beat up some journalists.

The prime minister met Sunday with top advisers at his residence. In a news release, Sharif said he would convene a joint session of parliament on Tuesday to address the crisis.

His government "condemned the attack on symbols of the state by two political parties," saying the violence was "undemocratic and unconstitutional."

"The steps taken by police and security forces to defend and defeat such acts were appreciated," the statement said.

The government blamed the protesters, saying officials had tried to negotiate. The statement called on Khan and Qadri to come back to the negotiating table, but they showed no inclination to do so.

Both Khan and Qadri, who spent most of the day inside the shipping containers that they have been living in for days, addressed their supporters in impassioned speeches.

"Now, it is death or freedom," Khan said. "We will not leave from here without Nawaz Sharif's resignation."

Both Khan and Qadri called on more people to come into the streets to support them.

But there were some apparent cracks in Khan's party. Javed Hashmi, president of Khan's Pakistan Tehreek-e-Insaf party, said at a news conference that the party had earlier decided not to push toward the premier's house and warned that the country was close to martial law.

The protests began with a march from the eastern city of Lahore on Pakistan's Independence Day, Aug. 14.

The demonstrations signify the starkest threat to Sharif's third term as prime minister. His previous term ended in 1999 with a military coup and his eventual exile. This turn in office has seen equally contentious relations with the country's powerful army. He's clashed with the military over the prosecution of the former army chief for treason, accusations the country's powerful spy agency was behind the attempted killing of a top TV anchor and a military operation in the tribal areas.

Sharif vowed Saturday that he would not step down, but if the violence continues, it could severely undermine his authority.

"The biggest question: Can Nawaz Sharif survive? The answer, in these frantic hours, must surely be a miserable, despondent no," read an editorial in Dawn, one of the country's leading English-language newspapers.