The party of Pakistan's former PM Benazir Bhutto has said more than 700 members were arrested overnight ahead of a planned mass rally on Friday.
They were held under emergency measures brought in on Saturday by President Pervez Musharraf, the party said.
Separately in Karachi, five more people were charged with sedition, which can carry the death penalty.
The moves came hours after US President George W Bush told Gen Musharraf in a "frank" phone call to hold polls soon.
Mr Bush told Gen Musharraf he could not be both army head and president.
Gen Musharraf has said he declared the state of emergency because of a crisis caused by militant violence and an unruly judiciary.
'Suicide attacks'
Ms Bhutto's Pakistan People's Party (PPP) is planning a huge public rally in Rawalpindi, close to the capital, Islamabad, on Friday.
Benazir Bhutto wants elections to be held on schedule
Party spokeswoman Farzana Raja told Reuters news agency members had been taken from their homes in mass detentions overnight in central Punjab province. The government has denied mass arrests.
Ms Bhutto had earlier insisted Friday's rally would go ahead despite an official ban and called for a "long march" next week unless Gen Musharraf changed course.
She is insisting that he restore the constitution, hold elections and resign as head of the army.
Ms Bhutto is also demanding the release of lawyers, judges and activists detained in the past few days.
The authorities have warned that police will not allow the demonstration to go ahead.
Rawalpindi mayor Javed Akhlas said: "We will ensure that they don't violate the ban on rallies, and if they do it, the government will take action according to the law."
"You can't be the president and the head of the military at the same time"
He told the Associated Press there was a "strong threat" of another suicide bomb attack against Ms Bhutto, who survived an assassination attempt in Karachi on 18 October that killed more than 140 people.
Protests since emergency rule was brought in have been limited but correspondents say the Rawalpindi rally could raise the stakes dramatically in the country's political crisis.
However, the BBC's Barbara Plett in Islamabad says other major opposition parties have not responded to Ms Bhutto's call for united action.
The five new people in Karachi charged with sedition had allegedly made comments and distributed leaflets against emergency rule.
Hasil Bizinjo and Ayub Qureshi, two leaders of a Baloch grouping, the National Party, Yusuf Mastikhan of the National Workers Party and union leaders Farid Awan and Yusuf Sahi were formally charged on Thursday and remanded in custody for two weeks.
A lawyer for the Baloch politicians, Arif Mohammad Khan, told the BBC the police report charged them with raising anti-government slogans and inciting people to violence in a gathering outside the Karachi Press Club on Tuesday.
Eight lawyers who are absconding, including one woman, were charged with sedition in the city on Wednesday.
'Indispensable ally'
Mr Bush said he had spoken to Gen Musharraf by phone for 20 minutes.
"My message was that we believe strongly in elections and that you ought to have elections soon and you need to take off your uniform," Mr Bush told reporters.
"You can't be the president and the head of the military at the same time, so I had a very frank discussion with him."
Mr Bush also noted that Gen Musharraf had been an "indispensable ally".
On Thursday, Pakistan's attorney general, Malik Mohammad Qayyum, quoted by AFP news agency, said elections scheduled for January would be held in February.
However, there have been a number of conflicting comments on the date of elections since the emergency was announced.
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