Story highlights Police fire tear gas after thousands of protesters threaten to march on PM's house

Nawaz Sharif said in a statement that he won't resign

Protesters have been in Islamabad for the last two weeks

Pakistani Prime Minister Nawaz Sharif is vowing to stay put despite protests against his government that have enveloped the nation's capital.

On Saturday, police fired tear gas on protesters after thousands of them threatened to march on the Prime Minister's house in Islamabad. At least 158 people were admitted to the Pakistan Institute of Medical Sciences, the hospital said. A 24-year-old man was in critical condition after being hit in the head by a rubber bullet.

After failing to negotiate a solution with the protest movements, Sharif on Thursday asked the country's powerful military for help brokering an end to the crisis.

"Tear gas is a normal practice undertaken all over the world to disperse a crowd. It is something that is an alternative to using force," Pakistan Defense Minister Khwaja Asif told CNN. "The situation had precipitated to a point that if action had not been taken then it would have been a free fall for the government."

At least 8,000 people have rallied in the city's center after allegations of vote-rigging during last year's election. Negotiations between Sharif's government and his opponents, some of whom are calling for his resignation, have reached an impasse.

Sharif announced in a statement on Saturday that he will not resign -- a demand he has called "unconstitutional."

Leading the two week-long protests are Imran Khan -- an enigmatic former cricket star who leads one of Pakistan's largest political parties -- and outspoken cleric Tahir ul Qadri, an outspoken cleric who wants to overhaul the country's political system.

Khan is demanding new elections, while Qadri is demanding much more sweeping reforms.

As the protests have grown in size and scope, the army has stationed personnel at government buildings in case protesters try to occupy them. A spokesman for Qadri said that protests tried to enter the road leading to the Prime Minister's residence.

Asif said that no decision had been made on whether to arrest Khan or Qadri.

"They violated their original commitment with the government to protest in a designated space like it is done in civilized countries," Asif said. "We cannot trust them."