Pakistani police have fired tear gas and scuffled with stone-throwing supporters of defiant former Prime Minister Imran Khan as they gathered for planned marches Wednesday toward central Islamabad for a rally he hopes will 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.

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.

A dozen demonstrators and several policemen were injured. Altercations between the police and Khan’s supporters were also reported elsewhere.

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.

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 has urged his supporters to remove the containers that were filled with earth and circumvent any blockades in order to enter the city. “I will be among you Wednesday afternoon,” he had vowed on Tuesday.

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Thousands of Khan’s supporters along with leaders of his Tehreek-e-Insaf party had already 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. He congratulated his countrymen for rejecting the rally by not participating in it and apologized for the inconvenience caused to citizens 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,” he told a news conference Wednesday. “We are fully prepared to handle him.”

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

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

Meanwhile, Pakistan’s Supreme Court was hearing a petition Wednesday to remove the blockades into Islamabad. Authorities say that if Khan submits a written assurance that his rally will be peaceful and confined to a public park, the government could consider lifting its ban.

The court was expected to announce an order about Khan’s rally later in the day.