Pakistan's civilian government failed on Tuesday to persuade opposition leader Imran Khan to ditch plans to march on the capital in protest against alleged ballot rigging in last year's general election.

Khan, a cricketer-turned-reformist politician who wants the government to resign and new elections to be held, plans to lead the mass demonstration from the eastern city of Lahore to the capital Islamabad on Thursday.

Fearing chaos, the government has banned the demonstration, but with a showdown looming, Prime Minister Nawaz Sharif announced that Supreme Court judges would investigate Khan's accusations.

"I respect the right to protest within the limits of the law, but no one will be allowed to destabilise the country," Sharif said during a televised address to the nation. His proposal was seen as an olive branch to the opposition, but Khan dismissed the offer, saying he could not trust any commission as long as Sharif remained in power.

"Whatever happens, we will march on August 14. If you stop us, you will be digging your own grave," he told reporters, referring to Sharif. "Before the judicial commission sits down to begin its work, Nawaz Sharif has to resign."

Khan's uncompromising stance is likely to unnerve the ruling party. Many in the government worry that Khan may be backed by individuals within the powerful military who are looking to weaken the civilian government and discourage it from pursuing policies they disapprove of.

Khan denies the accusation and the military denies meddling in politics. However, the nuclear-armed country of 180 million has a history of coups, protests and violent political rivalry.

Ties between the government and military were particularly strained by the last year's prosecution for treason of Pervez Musharraf - a former army chief who seized power from Sharif during his previous term as prime minister in a 1999 coup.

Activist cleric Tahir ul-Qadri also called a large protest rally for Lahore on Sunday. But following a police crackdown, he urged supporters to hold smaller protests in their hometowns instead and announced he would join Khan's march.

Police say they have arrested more than 500 of Qadri's supporters over the past few days, with clashes between Qadri's supporters and police breaking out in several cities.

Sharif's landslide election win last year marked the first time one elected government had handed power to another since independence from Britain in 1947.