Her separate telephone conversations with President Asif Ali Zardari and main opposition leader Nawaz Sharif marked the most senior foreign intervention yet in the crisis.

Clinton reassured the embattled Zardari about continued US support for his democratic government, the Pakistani presidency said.

"Mrs Clinton also discussed the prevailing situation in Pakistan and said the US was keen to see a stable and democratic system strengthened in the country," a statement said.

"The president thanked Mrs Clinton for US support to Pakistan," it added.

Siddiqul Farooq, a spokesman for the Pakistan Muslim League-N, told AFP that the US Secretary of State called Sharif and "discussed the current situation," without providing any further details.

A US embassy spokesman in Islamabad said he could neither confirm nor deny meetings or conversations.

Top US and British diplomats have intervened personally with leaders in Pakistan -- a key ally in the "war on terror" -- in recent days, urging the parties to resolve differences through negotiations, officials have said.

Thousands of lawyers and opposition workers plan to hold an anti-government protest in Islamabad to demand that Zardari act on promises to reinstate judges sacked by former military ruler Pervez Musharraf.

The government has arrested hundreds of people, banned rallies and cordoned off key roads in main cities to prevent the march from reaching Islamabad.