Clashes erupted today between ruling PML-N activists and supporters of opposition leader Imran Khan after shots were fired at his vehicle, as tens of thousands of protesters marched towards the capital with an aim of ousting Pakistan Prime Minister Nawaz Sharif.

Two opposition groups, led by cricketer-turned-politician Khan and Canada-based cleric Tahirul Qadri, plan to converge on Islamabad to press Sharif to call an early election little over a year after his landslide victory in the polls.

"The PML-N workers opened fire on my container (vehicle). If I was hit who could stop my party workers from reacting," said Khan, the chief of Pakistan Tehreek-e-Insaf (PTI) party.

He urged his party workers to remain peaceful, saying if the martial law was imposed in the wake of the government's actions his party would be blamed.

"The government is blaming us for the Gujranwala incident. It will also blame us in case martial law is imposed on such acts of the government," said Khan, who boarded a bullet-proof car after gun shots were reportedly fired at his vehicle in Gujranwala.

Khan also presented video footage of PML-N workers attacking his vehicle. The footage showed PML-N workers pelting stones and shoes on his vehicle with those on board seen ducking.

PTI spokesperson Shireen Mazari said at least eight party workers were injured in clashes in Gujranwala.

Three of them have been seriously injured and shifted to a hospital. They have suffered head injuries, she said.

Mazari alleged that Federal Commerce Minister Khurram Dastgir's men were involved in the incident. Gujranwala district is Dastgir's constituency.

"Police present there did not act to stop the PML-N activists," she alleged, adding that "we cannot give guarantee of our party workers remaining peaceful if they are attacked en route to Islamabad."

In another significant development, the Pakistan Supreme Court today issued an order against any unconstitutional step to remove the civilian government as anti-Sharif protests threatened political instability and possible military intervention in the coup-prone country.

The anti-government protesters of both the groups plan to stage a sit-in in Islamabad to press their claims, at the end of a "long march" -- which set off on a 370-km journey to the country's capital yesterday.

Khan initiated his 'Azadi March' from his Zaman Park residence in Lahore, while Qadri launched his 'Inqelab March' (revolution march) from the Model Town area of the city.

Khan has already underlined his demands, saying Sharif should resign and a caretaker government should be formed for holding a fresh general election in the country.

Qadri has also unveiled the goals of his 'revolution' march, saying its fundamental purpose is to restore democracy and alleviate poverty.

Khan's convoy had reached Gujrat, about 175 kilometers from Lahore, while Qadri's convoy had reached Sohawa, about 80 kilometers from Islamabad.

There were conflicting reports as to how the clash in Gujranwala started.

Punjab Law Minister Rana Mashhood said the PTI workers attacked the office of the PML-N which led to the clash.

"Our four workers have also been injured and we have closed all PML-N offices on the protest route. We have doubled the police force on the route of the long march of Mr Khan after this incident. We have also asked our party workers to stay calm," Mashhood said.

Another senior leader of Sharif's PML-N said there are several local offices of their party on the main road in the Gujranwala city and problem started when PTI workers chanted slogans against the Prime Minister while passing an office.

"Police took timely action to control the situation and we have asked workers to immediately close down all offices on the road," he said.

However, PTI's senior leader Shah Mahmood Qureshi said their protest rally was attacked by Sharif's supporters with stones.

Khan has also accused Sharif for the clash between their supporters. The Gujranwala city is considered a stronghold of Sharif.

Khan said after the incident that he will file a case against Sharif and his younger brother and chief minister of Punjab Shahbaz Sharif for any harm to his workers or party leaders.

Former minister and Awami Muslim League leader Sheikh Rasheed Ahmad, who is supporting Khan, said he also saw a person firing a pistol.

Khan's rally has moved very slowly and is expected to arrive in the capital late tonight.

Qadri's convoy has overtaken Khan and his supporters have been peaceful with no incident of violence reported during their journey.

Both Khan and Qadri had initially planned to take out a joint rally to oust Sharif but differences cropped between the two with Khan leaving his residence without waiting for Qadri which has angered the fiery cleric.

Qadri is now planning to hold a separate protest at Zero Point area of the capital while Khan's workers have been allowed to install a stage at Aabpara Chowk in Islamabad.

Some of Khan's supporters have already reached Islamabad and are waiting for his arrival. The government has removed all road blocks to facilitate the marchers entry in to the capital. But thousands of police and paramilitary soldiers have been deployed to maintain law and order.