Pakistani police detained hundreds of members of the former ruling party in the central city of Lahore on Thursday, weeks before the country heads to the polls, party officials said.
No official reason was given but the party, the Pakistan Muslim League-Nawaz (PML-N), says it is an attempt to stifle its ability to launch street protests.
A police official who asked not to be identified said more than 100 PML-N workers had been detained.
"We received instructions to take prominent PML-N activists from their areas to avoid a law and order problem," the official told Reuters.
The crackdown on the PML-N, whose leaders have accused the military of attempting to destabilise the party, comes a day before the party founder, ousted Prime Minister Nawaz Sharif, is due to return to Pakistan where he faces arrest.
Muhammad Mehdi, the party's media coordinator, told Reuters that nearly 500 workers had been detained in a "massive crackdown", including raids on the homes of a number of some of the party's local councillors.
PML-N members have said the crackdown on workers who handle street level organising will make it harder for them to stage rallies and protests on Sharif's return, when he will challenge the verdict and sentence against him.
A Pakistani accountability court on Friday sentenced Sharif in absentia to 10 years in prison for corrupt practices linked to his family's purchase of upscale London flats.
Sharif has denounced the court proceedings as politically motivated and a judicial witch-hunt, often suggesting the military was to blame.
Sharif has also said the military's intelligence wing, the Inter-Services Intelligence agency, is intimidating his party's candidates to switch loyalties, or to run as independents, ahead of the July 25 poll.
The run-up to the general election has been marred by accusations that the military is meddling in politics and muzzling the media to help usher cricketer turned politician Imran Khan's Pakistan Tehreek-e-Insaf (PTI) into power.
The military, which has ruled the nuclear-armed country for almost half its history and ended Sharif's second stint in power in 1999 in a bloodless coup, denies involvement.
Khan has also denied colluding with the military.
The Punjab government's caretaker law minister, Shaukat Javed, told Reuters his office had not ordered the detentions.
"Punjab government has not issued any kind of instructions to police for a crackdown against PML-N activists," he said.
Dozens of PML-N activists were arrested earlier this week and appeared in court on Wednesday, accused of plotting unrest to coincide with Sharif's return.
A PML-N official told Reuters that police began monitoring his house and the residences of a number of other party activists on Wednesday night.
"I left my house last night and am hiding at a friend's house," Mustafain Shah said.