British police have arrested two more suspects in an investigation into “Islamist-related terrorism”, a day after nine were detained, including a top radical preacher.

The two men “were arrested in the early hours of this morning in a vehicle on the M6 motorway”, which runs between central and northern England, police said in a statement on Friday.

One of them, aged 33, was arrested for “encouraging terrorism” and “being a member of a proscribed organisation”, while the other, 42, was held for “assisting an offender”.

• Mental health key to tackling terrorism 
• Australia’s new anti-terrorism laws explained

“These arrests and searches are part of an ongoing investigation into Islamist-related terrorism and are not in response to any immediate public safety risk,” police said.

The nine arrested on Thursday are also suspected of links to a banned organisation, reportedly the extremist Islamist group Al-Muhajiroun.

It was co-founded by one of the detained, Anjem Choudary, who is regularly quoted in British tabloids for his controversial views on Islam and the Middle East.

Choudary, a 47-year-old former lawyer, is also known for staging an anti-West march through the town of Royal Wootton Bassett at a time when it was known for honouring British soldiers killed in Afghanistan as the coffins returned home.

Al-Muhajiroun has had a string of different incarnations over the years and aims to overthrow the British government and establish a global Islamic caliphate, according to the London-based International Centre for the Study of Radicalisation and Political Violence.

Meanwhile, Spain and Morocco on Friday arrested nine suspected members of a group in north Africa linked to the violent Islamic State jihadists, the Spanish government said.

Spanish police and Moroccan security forces “today detained nine members of a terrorist cell … linked to the terrorist organisation Islamic State”, Spain’s interior ministry said.

Authorities in various countries have voiced fears about foreign fighters flocking to join the IS organisation, which controls parts of Syria and Iraq.

The nine suspects were seized in Melilla, a Spanish territory on the northern tip of Morocco, and the nearby Moroccan city of Nador, the ministry said in a brief statement.

It said investigations were continuing and the ministry would give further details later.

Moroccan authorities estimate there are between 1500 and 2000 Moroccan jihadists fighting in Syria and Iraq. Spain has arrested dozens of suspected extremists in raids this year.

A US-led alliance has launched air strikes against the IS militants in Syria and Iraq.

IS fighters have beheaded a British aid worker, David Haines, two US journalists and are holding two other Britons, Alan Henning and John Cantlie.