Pakistani opposition politician Imran Khan has called for a campaign of civil disobedience as he addressed thousands of supporters protesting for a second day against the government of Nawaz Sharif in Islamabad.

Khan, who heads parliament's third largest political bloc, on Sunday called on Pakistanis to refuse to pay taxes and utility bills since "the corrupt government was stealing the people's money", and told his supporters that their protests should be without bloodshed.

On Saturday, Khan demanded Sharif resign within 48 hours or face protests in the capital's high security "Red Zone" of government buildings and embassies. On Sunday he said that his supporters should not resort to violence, but that he could not guarantee what would happen if his ultimatum was unheeded.

His address was part of a joint protest against the government by Khan and Tahir ul-Qadri, a Muslim religious leader and opposition politician, known as the "long march" that set off on Thursday from Lahore, about 300km from the capital.

However, only a few thousand protesters remained in the capital on Sunday - far short of the million promised by Khan and Qadri. They faced 30,000 police and other security forces on the streets, the AFP news agency said.

Khan claims the government came to power fraudulently in last May’s general election, which was described by international monitors as free and credible.

Qadri claims that Sharif and his younger brother, Shahbaz Sharif, the chief minister of Punjab, have no right to sit in government.

He also claims Shahbaz’s complicity in the deaths of 10 workers of his Pakistan Awami Tehreek Movement in clashes with police at his headquarters in Lahore on June 17.

Shabaz denies any knowledge of the police operation and has ordered a judicial commission to investigate.