Altaf Hussain, the exiled leader of Muttahida Qaumi Movement, one of Pakistan's biggest political parties, was arrested on Tuesday by Scotland Yard in London over his anti-state speeches, especially the one in 2016 in which he said Pakistan was the "epicentre of terrorism" and "a cancer for entire world".
The Metropolitan Police would only confirm that a man in his 60s had been held in an investigation into speeches related to his MQM party.
The MQM has dominated politics in Karachi for three decades because of its support in the densely populated working class neighbourhoods of Urdu-speaking Muhajirs, descendants of Muslims who migrated from India when Pakistan was created in 1947.
Hussain, 65, requested asylum in the 1990s and later gained United Kingdom citizenship.
But he maintains a firm grip over the MQM and its main power base, the financial capital of Karachi.
Following his arrest, Karachi city police chief Amir Ahmed Shaikh issued directions for stepped up patrolling and vigilance with anti-riot squads and equipment in Karachi.
Hussain was arrested on Tuesday and taken to a south London police station. Fifteen officers took part in a dawn raid at his north London home, Geo News reported.
Altaf Hussain has been arrested in relation to the hate speech of 2016 in which he had urged his followers to take the law into their own hands, it said.
Hussain had delivered a fiery speech on August 22, 2016, after which party workers vandalised a media office in Karachi and chanted anti-Pakistan slogans.
The MQM supremo had not only raised slogans against Pakistan but also called the country "a cancer for entire world".
"Pakistan is cancer for entire world,” he had said.
"Pakistan is headache for the entire world. Pakistan is the epicentre of terrorism for the entire world. Who says long live Pakistan...it's down with Pakistan," Dawn newspaper reported, quoting him as saying in 2016.
His arrest was confirmed to the BBC Urdu service by his spokesman, Qasim Raza.
The UK police statement does not name Hussain, referring instead to "an individual associated with the Muttahida Qaumi Movement in Pakistan".
He was arrested at an "address in north-west London... on suspicion of intentionally encouraging or assisting offences contrary to Section 44 of the Serious Crime Act 2007", the statement said, adding that the man remains in custody.
Two premises are being searched, in an investigation led by the Met's Counter Terrorism Command.
It focuses on "a speech broadcast in August 2016 by an individual associated with the MQM movement in Pakistan as well as other speeches previously broadcast by the same person".
Over 20 identical First Information Reports pertaining to 'hate speech' were registered against Hussain and several other leaders and workers of the MQM's London and Pakistan factions, Dawn newspaper reported.
The London Met Police which began investigating Hussain's 2016 speech visited Islamabad in April this year to collect evidence and interview key witnesses.
Following Hussain's August 2016 speech, a number of cases were filed against the party chief in Karachi, Quetta and Gilgit-Biltistan region. Anti-terrorism courts hearing these cases have also issued separate non-bailable warrants against the MQM chief.
He is wanted in various cases that include charges of terrorism and incitement to violence.
In October 2016, the Scotland Yard had cleared him in a money laundering case, citing "absence of adequate evidence".
Hussain's UK media office, calling itself the MQM Secretariat, issues regular communique against the Pakistani government.
In its last release on May 15, it pointed to Hussain's social media intervention warning top authorities at the International Monetary Fund to be "cautious" before signing any agreement with "unreliable countries like Pakistan".