Nawaz Sharif, leader of the main opposition PML-N party, has called upon Pakistanis to rise up against the government, which is viewed as failing on political, economic and security fronts.
But authorities would also step up their efforts to stop the protests, Kamal Hyder, Al Jazeera's correspondent in Islamabad, said.
"Large-scale arrests have continued in Pakistan and there is a likelihood that there will be even more arrests."
Lawyers, opposition parties and civil activists are undertaking a so-called long-march of 1,500km from the city of Karachi to Islamabad to demand that Asif Ali Zardari, the president, reinstate judges sacked by his predecessor Pervez Musharraf.
Sharif's PML, whose main base is in Punjab, also supports the reinstatement of the judges.
The march, which began on Thursday, is occurring despite a ban on rallies in Punjab, which surrounds Islamabad and is the nation's most important political constituency, and Sindh, where Karachi is located.
Our correspondent said the protests are adding to Pakistan's difficulties.
"The political crisis in Pakistan is coming to a boiling point and that has prompted the international community ... to speak to the Pakistan leadership and try to avert a major political crisis within the country," he said.
"For Pakistan it is a crisis it could do without because of the economic crisis and other political issues that still confront the country."
Speaking to Al Jazeera, Talat Masood, a retired Pakistani general, said: "[This] is one of the most serious [political crisis] that Pakistan has witnessed in the last 61 years, because it has so many implications.
"Firstly, the government which came with a popular mandate, is no longer popular. In fact the popularity of President Zardari is less than 10 per cent.
"What is worse is that his own party is today factionalised and it is imploding from the inside.
"At the same time the military is not supporting him. And he is behaving very authoritative and dictatorial, so it makes matters worse."
Masood said that there was huge support for the opposition, whose parties have drawn together.
"Even if you suppress it there is no victory in this, because this movement will not go away," he said.
Against this backdrop of rising political tension, Richard Holbrooke, the US special envoy to Pakistan and Afghanistan, and Anne Patterson, the US ambassador to Pakistan, spoke to Zardari by telephone on Thursday.
The US stressed the need for violence to be avoided, according to the state department.