LONDON — When British police officers mounted a dawn raid on a calm London suburb on Tuesday, with orders to arrest the powerful Pakistani political boss Altaf Hussain, the reverberations were felt most intensely 4,000 miles away in the port city of Karachi.

Businesses hastily shuttered, trains stopped and workers raced home, clogging the streets in chaotic traffic jams, amid panic that the arrest of Mr. Hussain, a charismatic figure who for the past two decades has controlled Karachi from his exile in England, would result in an eruption of political bloodshed.

The sight of a sprawling megalopolis of 20 million people so visibly girding itself for trouble was a measure of the power that Mr. Hussain, through his Muttahida Qaumi Movement party, has come to wield over Karachi — a vital economic hub long divided by violent factional competition, more recently threatened by Taliban infiltration, and suddenly seized by trepidation over what will happen now that Mr. Hussain is in the custody of London’s Metropolitan Police.

“This is potentially very serious,” said Abbas Nasir, a former editor of the Pakistani newspaper Dawn. “The removal of someone as powerful as Altaf Hussain is always going to leave a vacuum. His party is in for a challenging time.”