ISLAMABAD - Pakistan arrested hundreds of opposition activists Wednesday and banned protests in two regions ahead of a planned rally outside parliament that could weaken the already shaky rule of the government.

The crackdown by police and intelligence agencies stands to damage the democratic credentials of the ruling party of President Asif Ali Zardari and stoke further anger at the government's one-year-old rule. The opposition, along with lawyers spearheading the planned protest, vowed to press on.

"I cannot rest when Pakistan is being taken toward disastrous circumstances," Nawaz Sharif, the head of the largest opposition party told several thousands supporters at a rally in the North West Frontier Province. "We cannot compromise when all institutions are ruined and the system is on the verge of collapse."

The upheaval comes as nuclear-armed Pakistan is grappling to contain surging violence by al Qaeda and the Taliban and fix an economy that functions only because of support from international lenders. It could lead to a political deadlock and even some form of intervention by the country's powerful military, which has often seized power in the past following chaotic civilian rule.

Pakistan's feisty lawyers, Sharif's party and two other smaller groupings are demanding Zardari fulfil a pledge to restore a group of judges fired by former president Pervez Musharraf in 2007. Zardari is refusing to do so, apparently fearing they could try to limit his power or reopen corruption cases against him.

Last month, the Supreme Court banned Sharif and his brother from elected office, enraging their followers and energizing the protest movement further.

The protesters have vowed to gather in cities around the country Thursday before leaving for the capital, Islamabad, where they plan to stage a sit-in at the parliament building until their demands are met.

But public gatherings were banned in Punjab, Pakistan's most populous province and Sharif's stronghold, and Sindh, home to the country's largest city of Karachi, the home secretaries in both provinces said.

Rao Iftikhar, the home secretary in Punjab, said "around" 300 political activists had been arrested there under a law that allows for six months imprisonment. Past governments have often rounded up activists to weaken demonstrations and released them after a short time.

Aitzaz Ahsan, a lawyer's movement leader, said he believed "hundreds" of lawyers had been arrested, but insisted the movement -- which helped bring down the government of Musharraf -- would not falter.

"How long can the state resist?" he told a local TV station. "We will keep on knocking on the door of Islamabad relentlessly."

Zardari was visiting Iran on Wednesday. Meanwhile, army chief Gen. Ashfaq Parvez Kayani met with Prime Minister Yousuf Raza Gilani, according to a press release from the premier's office.

The U.S., which supported Musharraf but is backing the civilian government, has yet to make any public statement on the crisis. Last week, Britain appealed for political unity, saying the bickering was distracting Pakistan from the "mortal threat" posed by al Qaeda and the Taliban.