Pakistan opposition warns Imran Khan arrest would be 'red line'

Pakistan opposition leaders warned that authorities would cross a "red line" if they arrested former prime minister Imran Khan after he was reported under the anti-terrorism act for comments he made about the judiciary.

Since being ousted by a vote of no confidence in April, Khan has held mass rallies across the country, warning state institutions including the military not to back the coalition government led by his long-time political rival Shehbaz Sharif.

Hundreds of people gathered outside Khan's house; but the former leader has been fighting a raft of charges for months and has so far not been arrested.

A First Information Report was filed with police -- the first step of a process that could lead to formal charges and arrest.

Lawyers for Khan preemptively applied for pre-arrest bail at the Islamabad High Court , which was granted until at least Thursday.

There was a low-key police presence outside Khan's residence, with around 500 party supporters gathered in the affluent suburb of Bani Gala.

In a statement, Khan's Pakistan Tehreek-e-Insaf party (PTI) said the latest accusations against him were "frivolous".

"We have serious reservations on this politically motivated move which leads towards further instability in the country," it said.

He criticised a magistrate responsible for keeping a PTI official in police custody, after party leaders say he was tortured in detention.