* Weeks of anti-government protests demanding Sharif resign
* Violence erupts late on Saturday, police fire tear gas
* Army holding emergency meeting to discuss crisis (Adds Khan's quotes, details)
By Syed Raza Hassan and Mehreen Zahra-Malik
ISLAMABAD, Aug 31 (Reuters) - Pakistani opposition leader Imran Khan ordered his supporters on Sunday to take to the streets and stand up against security forces after at least three people were killed in clashes between protesters and police in the capital overnight.
Pakistan has been shaken by weeks of anti-government demonstrations, with violence erupting late on Saturday after thousands tried to march on Prime Minister Nawaz Sharif's residence, prompting police to fire tear gas to stop them.
Demonstrators demanding Sharif's resignation have camped outside government offices for more than two weeks but it was the first time violence broke out as protesters, some armed with sticks and wearing gas masks, tried to break through police lines.
The eruption of violence has unnerved many in the coup-prone country, with Sharif looking increasingly cornered amid relentless calls by the opposition for him to step down.
Small skirmishes continued into Sunday, with police occasionally firing tear gas canisters but no major acts of violence were reported. The atmosphere remained tense, however, with thousands of people massing outside parliament.
Khan, an outspoken cricketer-turned-politician, told his supporters in central Islamabad he would not back down from his demand for Sharif to resign and called on more protesters to join him.
"I am prepared to die here. I have learnt that government plans a major crackdown against us tonight," he said. "I am here till my last breath."
Raising the possibility of another night of clashes, Khan told the cheering crowd to directly challenge security forces protecting the parliament and the prime minister's house.
"The way you stood up last night, you have to stand up today also," he said. "We will face them and make them run away this time."
At least three people were killed and 200 wounded overnight, hospital officials said.
The violence broke out despite the army's public intervention in the conflict. How the crisis unfolds ultimately lies in the military's hands in a country ruled by generals for half of its entire history.
Highlighting the urgency of the situation, army chiefs held an emergency meeting in the nearby garrison city of Rawalpindi on Sunday night to discuss the crisis, prompting speculation that the army could take decisive action to end the crisis.
Sharif, who swept to office in the country's first democratic transition of power last year, has firmly resisted opposition calls for him to resign while agreeing to meet their other demands such as an investigation into alleged fraud during last year's election.
His office reiterated on Sunday evening that his resignation was out of the question.
Himself ousted in a coup in 1999 during an earlier stint in office, Sharif still has a difficult relationship with the army. Even if he survives this crisis, he will remain significantly weakened and sidelined on key issues such as foreign policy and security.
Opposition leader and cleric Tahir ul-Qadri, thousands of whose supporters have rallied alongside Imran Khan, said earlier protests would not subside unless Sharif resigned.
"State atrocities have reached their peak," he told his supporters from atop a shipping container. "Imran khan and Dr Qadri are fighting this war together." (Additional reporting by Sheree Sardar; Writing by Maria Golovnina; Editing by Hugh Lawson)