Pakistan's main opposition leader Imran Khan says the authorities are acting outside the law in their attempt to arrest him.

"They should all be subject to the law of the land," he told the BBC.

Police have clashed with supporters of Mr Khan, a former prime minister, outside his residence as they tried to detain him on a court order.

He is facing allegations he sold state gifts while in office. He says the case is politically motivated.

There was a tense situation outside his compound in the city of Lahore on Tuesday as police tried to force their way in.

Officers fired tear gas shells to disperse supporters of Mr Khan, who threw stones and bricks.

In a video address, the politician, 70, offered to sign a guarantee that he will appear in an Islamabad court on Saturday.

He added that there were security concerns as two militant attacks had previously taken place at the court.

Mr Khan claimed there was "no reason" for police to arrest him on Tuesday as he had taken protective bail until Saturday. He also said the government was determined to put him behind bars after previous failed attempts.

"I'm mentally prepared that I'm going to be spending my night in a cell," he said on Tuesday. "I don't know how many nights, but I'm all prepared for that."

The former prime minister showed what he said were tear gas canisters, which he said had been fired into his property.

He said the authorities were trying to arrest him to stop his party from taking part in forthcoming elections. But he added: "Whether I am in jail or not they will not be able to stop my party winning."

Government minister Marriyum Aurangzeb said the move had nothing to do with elections and police were only complying with court orders to arrest him over corruption cases. 

She claimed that Mr Khan was using his party workers, women and children as human shields to evade arrest and stoke unrest.

Mr Khan's supporters are also holding protests in other cities.

He was ousted as prime minister last April but has kept up pressure on his successor Shehbaz Sharif with demonstrations and speeches calling for elections due later this year to be held early.

He blames Mr Sharif for an assassination attempt at a rally in November in which he was wounded in the leg.