Pakistani police fired tear gas and clashed with supporters of Imran Khan as they stopped a rally organized by the former prime minister from reaching Islamabad.
Khan had called on his supporters to hold a rally outside parliament and stay there until a date for fresh elections was announced.
But the government of Khan's successor, Shahbaz Sharif, banned the rally and blocked routes in and out of Islamabad and other major cities in Punjab Province.
The Supreme Court later ruled that the rally could go ahead but only at a specifically allocated public grounds and on condition the demonstrators disperse after an address by Khan.
Khan insisted his rally would be peaceful. He traveled by helicopter on May 25 to a highway some 100 kilometers northwest of Islamabad, where he condemned the police crackdown and urged supporters to join the rally.
"We will remain in Islamabad till announcement of dates for dissolution of assemblies & elections are given," he said on Twitter.
The marches have raised fears of major violence between supporters of Khan and security forces.
Organizers planned for Khan supporters to travel by car and bus to Islamabad's city limits then march on foot from there.
Police sealed off the capital to stop protesters from entering the city and arrested activists as they tried to cross the barricades, local official Bakhat Nawaz said.
By late evening, Khan had not arrived in Islamabad, while police made arrangements for an alternate location for the rally far from the parliament building.
Earlier in the day, riot police fired tear gas and pushed back demonstrators who hurled stones as they tried to pass a roadblocked bridge near Lahore to board buses bound for Islamabad.
Thousands of Khan's supporters along with leaders of his Tehrik-e Insaf party massed in Peshawar, the capital of northwestern Khyber Pakhtunkhwa Province, where his party rules.
Small groups of his supporters clashed with riot police in Karachi, Lahore, and Islamabad, Interior Minister Rana Sanaullah said.
A mob torched a prison van in Karachi after clashing with police, and another group of protesters set fire to several trees along a main thoroughfare in Islamabad, officials said.
More than 1,700 Khan supporters were arrested in the crackdown, according to Sanaullah. Of the total, 250 were later freed, he said.
"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. "We are fully prepared to handle him."
Sanaullah said no one had been stopped from exercising his or her constitutional and legal right to hold a rally or take part in democratic politics, "but we can't allow anyone to sow violence and chaos."
Khan is a former cricket star who served as prime minister for more than three years until he was ousted last month in a no-confidence vote.
He has accused the United States of manipulating the opposition because of his warm relations with Russia and China. The United States has denied the allegations.
Meanwhile, talks between Islamabad and the International Monetary Fund (IMF) 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 Khans government in February, forcing the IMF to withhold a crucial tranche of about $1 billion.
Pakistan entered the three-year, $6 billion IMF deal in 2019 but is struggling to implement tough policy commitments.
Pakistani Foreign Minister Bhutto Zardari told Reuters on the sidelines of the World Economic Forum in Davos, Switzerland, that the context of the deal had changed since it was negotiated.
"This deal is a pre-COVID deal. It is a pre-Afghanistan fallout deal. It is a pre-Ukrainian crisis deal. It is a pre-inflation deal," Bhutto Zardari said, adding that it would be unfair and unrealistic to expect a developing country like Pakistan to navigate geopolitical issues under the current agreements.