The protesters are determined that the march should go ahead
Police in Pakistan have arrested dozens of lawyers and opposition activists ahead of an anti-government protest march due to begin on Thursday.
Political gatherings have also been banned in Sindh and Punjab provinces.
Protesters led by lawyers and opposition leader Nawaz Sharif want judges sacked by former President Pervez Musharraf to be reinstated.
Tensions are high after the Supreme Court banned Mr Sharif and his brother from holding elected office.
Opposition groups and lawyers have been mobilising support for the four-day march on Islamabad since the court ruling on 25 February. The Sharifs have blamed President Asif Ali Zardari for influencing the decision.
The government says the planned march is aimed at destabilising the country. Correspondents say it appears determined to keep protesters outside the capital, Islamabad.
The deteriorating political and economic situation and high-profile attacks by militants have heightened fears over Pakistan's future, just six months after President Zardari took office.
The marchers plan to stage an indefinite sit-in in Islamabad and there are fears there may be violent confrontations with security forces.
"Provincial governments are fully empowered to take action to keep law and order... There are security threats," Interior Ministry chief Rehman Malik told reporters.
The government has issued orders to place senior leaders of Mr Sharif's Pakistan Muslim League-Nawaz (PML-N) under house arrest. It is not clear whether there are also orders to place Mr Sharif and his brother Shahbaz under detention.
On Wednesday, Mr Sharif addressed a massive rally in the northern town of Abbotabad and urged Pakistanis to join the march to Islamabad.
"It is the duty of every Pakistani citizen to stand up for the restoration of the judiciary," he said.
"I am not doing this for the sake of personal power, but for the future of Pakistan. Mr Zardari has broken his word on the issue, and we have been left with no choice."
Earlier, Mr Sharif's brother, Shahbaz, said: "If this [civilian] government bans political gatherings, then I don't see how it is different from the military regime of Gen Musharraf."
The PML-N insists that it is a democratic right to hold rallies
He had to leave his post as chief minister of Punjab last month after the court decision declaring him ineligible to hold office.
Reports from several cities in Punjab province say that the police have arrested a large number of opposition workers to maintain public order.
A warrant has been issued for the arrest of another opposition leader, Imran Khan, the former Pakistan cricket captain.
Raids have also been conducted on the residences of prominent lawyers and opposition legislators, but most are reported to have gone into hiding to avoid arrest.
"We are avoiding arrest because we want to lead the march," a prominent lawyer, Aitzaz Ahsan, told a local television station.
Chief among the lawyers' demands is the restoration of a former chief justice of Pakistan, Iftikhar Chaudhry.
He was sacked along with some 60 other senior judges by Gen Musharraf in November 2007, sparking countrywide protests.
Among other things, the judges had been due to rule on a controversial amnesty covering Mr Zardari and his wife Benazir Bhutto, who was later assassinated.
Mr Zardari has in the past accused Iftikhar Chaudhry of complicity in a campaign to victimise him while imprisoned on what he says were trumped up murder and corruption charges in the 1990s and the early part of this decade.
Under Mr Zardari's administration, Pakistan is descending deeper into crisis, BBC correspondents say.
In the past the military, Pakistan's most powerful institution, has stepped in to seize power from failing governments.
The current army chief, Gen Ashfaq Pervez Kayani, has said he will respect civilian rule.