ISLAMABAD -- Pakistani police have fired tear gas and scuffled with stone-throwing supporters of ousted Prime Minister Imran Khan ahead of planned marches Wednesday toward central Islamabad. The defiant former premier had called followers to rally outside Parliament to bring down the government and force early elections.

The marches have raised fears of major violence between supporters of Khan - now Pakistan's top opposition leader - and security forces. The government of Khan's successor, Shahbaz Sharif, has banned the rally and warned Khan he could face arrest if he went ahead with the demonstrations.

The country's Supreme Court ruled later Wednesday that Khan's rally could go ahead - but only at a specifically allocated public grounds and on condition the demonstrators disperse after an address by the former prime minister. The court also asked Khan's lawyer, Babar Awan, to ensure that the rally remains peaceful.

However, Khan persisted, urging supporters to head toward the square near Parliament for the rally that he plan would evolve into a sit-in there until the government resigns. By late evening, the former prime minister had not arrived in Islamabad while police were making security arrangements for an alternate location for the rally, far from the parliament building.

The government also summoned troops to guatd important buildings, including the parliament and offices of the president and prime minister in Islamabad, according to a notification issued by the Interior Ministry. The measures come following clashes between demonstrators and police.

Earlier in the morning, riot police fired tear gas and pushed back hundreds of demonstrators who hurled stones as they tried to pass a roadblocked bridge near the city of Lahore to board busses bound for the capital, Islamabad. Dozens of Khan's followers also briefly clashed with police in Islamabad, where the demonstrators set fire to bushes lining a main boulevard, sending smoke and flames rising into the sky.

Altercations were also reported elsewhere, including in the port city of Karachi, where demonstrators burned a police vehicle.

At least a dozen demonstrators and several policemen were injured. Ahead of Wednesday's marches, authorities used dozens of shipping containers and trucks to block off major roads into Islamabad.

Khan, a former cricket star turned Islamist politician served as prime minister for over three and half years until last month, when he was ousted by a no-confidence vote in Parliament. Since then, he has held rallies with thousands of people across the country.

Khan says his removal from office was the result of a U.S.-organized plot and collusion with Sharif, whose government has vowed a stern response if Khan violates the ban. Washington has also denied any role in Pakistan's internal politics.

Despite the ban, Khan is insisting his rally will be massive and peaceful - and continue until the government agrees to hold fresh elections this year, not in 2023 as scheduled. Organizers had planned for crowds to travel by car and bus to Islamabad's city limits, then march on foot from there.

Khan himself traveled by helicopter to a highway some 100 kilometers (62 miles) northwest of Islamabad, where he condemned the police crackdown and urged supporters to join the rally.

“My message for the nation: Everyone must break out of the grip of fear to achieve freedom,†he wrote on Twitter, before starting out by vehicle from the Swabi interchange. His convoy still faces a series of roadblocks ahead that would require heavy machinery to remove.

Khan called on supporters to remove the earth-filled containers and circumvent any blockades in order to enter Islamabad. “I will be among you Wednesday afternoon,†he had vowed Tuesday.

Thousands of Khan's supporters along with leaders of his Tehreek-e-Insaf party massed in Peshawar, the capital of northwestern Khyber Pakhtunkhwa province where his party rules. From there, his followers must cross a bridge at the province's border that the government has blocked, before assembling on the outskirts of Islamabad.

The government launched a crackdown and arrested more than 1,700 Khan supporters, according to Interior Minister Rana Sanaullah. The minister lauded his countrymen for rejecting a massive rally and apologized to citizens for any inconvenience due to the blockades.

“Imran Khan had claimed that he would gather 2 million people here in Islamabad today, but he is marching toward Islamabad along with only 6,000 or 7,000 demonstrators,†Sanaullah told a news conference Wednesday. “We are fully prepared to handle him.â€

Authorities have deployed additional police and paramilitary troops on highways and in Islamabad, with tractor trailers parked across both lanes of traffic in several areas.

The measures were announced after a policeman was killed Tuesday during a raid on the home of a notable Khan supporter in Lahore.

In a separate development Wednesday, dayslong talks between Islamabad and the International Monetary Fund concluded in Qatar without Pakistan securing a revival of a $6 billion bailout package from the global lender.

After the talks, the IMF urged Pakistan to remove subsidies on fuel and energy. The subsidies were approved by Khan's government in February, forcing the IMF at the time to withhold a crucial tranche of about $1 billion.