Tight security marked the beginning of the action in the port city of Karachi, where the authorities arrested thousands of political activists and leaders on Saturday to prevent an anti-government rally.

The BBC's Owen Bennet-Jones: "Several politicians have been detained"

Witnesses said paramilitary troops and police were guarding sensitive parts of the province to prevent violence, and there were no early reports of any disturbances in Karachi, where traffic was seen on the roads.

Hunger strike

In Hyderabad, about 120km (72 miles) north of Karachi, legislators belonging to former prime minister Benazir Bhutto's Pakistan People's Party tried to stage a one-day hunger strike, in another show of protest.

Police sealed off the district where the protest was planned, and arrested about 300 people.

A rally was held at Lahore airport after police prevented alliance leaders flying to Karachi

The two-day strike in Sindh is the latest in a series of protest rallies by the newly formed alliance against Mr Sharif, who enjoys a two-thirds majority in the National Assembly and says the opposition movement poses no serious threat to his government.

Last week, Mr Sharif imposed a two-month ban on demonstrations in Sindh which has resulted in the police crackdown. The opposition has promised to defy the ban.

It blames the prime minister for the country's economic woes and for diplomatic blunders during a recent stand-off with India over Kashmir, which brought the nuclear rivals to the brink of a fourth war.

MQM leader Tariq Javed told Reuters news agency that the police crackdown on the opposition, activist arrests and heavy teargassing to disperse the rally on Saturday had given fresh impetus to their anti-Sharif movement.