The Pakistan Awami Tehrik (PAT) chief Tahir ul Qadri, the co-organiser of the protest, said his party's seven workers were killed and 75 others injured, while Imaran Khan, chairman of the Pakistan Tehrik-i-Insaf (PTI), said one of his workers was killed in clashes, Xinhua reported.
However, the spokesperson of the Pakistan Institute of Medical Sciences (PIMS), a government-run hospital, said no dead person was brought to the hospital.
The Federal Defence Minister Khawja Asif, however, denied any causality in the clashes, saying that it is exaggeration by the protesters.
Several media persons and policemen were reportedly among the injured.
Interior Minister Nisar Ali Khan said police used force after a group of demonstrators tried to enter an important governmental building to occupy it.
The government has ordered police to arrest both leaders as soon as possible to control the situation, the report said.
The clashes erupted after leaders of the two opposition parties the PAT and the PTI asked their workers about 10 p.m. to move from the parliament towards the PM house after two-week peaceful sit-ins in Islamabad.
Police used baton charge, fired tear gas shells and rubber bullets to disperse the thousands of the protesters, who were demanding the resignation of Prime Minister Nawaz Sharif.
PTI and Muttahida Qaumi Movement (MQM) have announced to observe a day of mourning Sunday following the clashes between protesters and law enforcers in Islamabad.
Following the clashes in Islamabad, party workers of the PTI and PAT started protests in other major cities, including Rawalpindi, Lahore, Faisalabad and Karachi.
Thousands of supporters of the two parties reached Islamabad Aug 15 and entered a high security area "Red Zone" Aug 19 and started sit-ins in front of the parliament.
Both parties held talks with the authorities on their demands but that finally ended in failure.
Khan, whose party has 34 seats in the lower house, alleged that the prime minister's party managed to win the 2013 general election through rigging.
He has been demanding for a free and fair probe into the rigging by a panel of the apex court, and the PM should step down till the time inquiry report comes.
According to an earlier report, at least one women protester was killed and over 50 were injured.