Thousands of anti-government protesters led by Nawaz Sharif, Pakistan's former premier, battled with police in Lahore yesterday as troops were put on alert before a planned national demonstration in Islamabad today.

The clashes came after Mr Sharif had been placed under house arrest in an effort to prevent him attending the Islamabad protest, which is seen as an open challenge to president Asif Ali Zardari. The authorities last week detained scores of opposition leaders, political activists and anti-government lawyers.

Mr Zardari faces opposition from two main areas: angry lawyers seeking reinstatement of Iftikhar Mohammad Chaudhry, former chief justice of the Supreme Court, and political rivals in Punjab province. He also has to confront the threat of an Islamist militant insurgency.