Police and protesters clash in Pakistan's capital

ISLAMABAD, Pakistan - Antigovernment 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 in clashes that started Saturday night and continued to boil over Sunday, officials said.

Prime Minister Nawaz Sharif met with top advisers seeking a way to ease the violence, which has raised the stakes in the two-week-old 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 tear gas. 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 around the parliament and in other parts of the city, police said, wielding shields taken from police, iron rods, batons, stones, and bricks.