Asked about its decision to do away with the posters, TfL told The Independent that it "did not comply with our advertising guidelines."

However, many think that it was more than that and Islamabad actually had a big role to play in it.

Later, it was reported that the British High Commissioner in Islamabad was summoned to appear before the Pakistani Foreign Secretary, who made it clear that the adverts 'directly attack its territorial integrity and sovereignty'.

A statement by the Pakistan Foreign office released by Pakistan Today read, "The High Commissioner was informed that Pakistan, in line with the UN Charter, rejects actions and advertisements with malicious content that impinge on our sovereignty and territorial integrity."

“Pakistan is aware of the intentions of such sinister and malicious campaigns, which should not be allowed on the soil of a friendly country,” it read.

For years, Baloch leaders have accused Pakistan of torturing the ethnic population of Balochistan.

It is a resource-rich province, forming the southwestern region of the country.

Prominent Baloch leader Bhawal Mengal told The Independent: "The bullying tactics of Pakistan are an attack on freedom of expression. They are an anti-democratic bid to censor the voice of the Baloch people and cover up the war crimes of the Pakistan army in Balochistan. This is a peaceful advertising campaign."

The adverts later appeared on London buses, much to their relief.

The campaign slogan reads, "Free Balochistan”, “Save the Baloch People” and “Stop Enforced Disappearances”.

But Mengal knows that the joy can be short lived.

He was quoted by Hindustan Times as saying, "We have delivered our message through taxis, billboards across London and now on over 100 buses. So far the buses are carrying the adverts but we can expect pressure again from Pakistan authorities."

Apart from London, Zurich is another prominent area from where several Baloch leaders and outfits operate and it seems Islamabad is forcing the Swiss government to suppress the activities.

On Thursday, Swiss authorities detained Mehran Marri, the representative of Balochistan at the UNHRC and the EU, and his children at the Zurich airport, prompting stern reactions from Baloch leaders.

"Detention and travel ban on @MehranMarri by Swiss authorities on bogus Pakistani allegations is disturbing. Mr. Marri has been regularly highlighting HR abuses in #Balochistan by state forces at UNHRC which irks Pakistan. When will the world understand the true face of Pakistan?" representative of Baloch Republican Party (BRP) at the UNHRC, Abdul Nawaz Bugti, tweeted.

"Weeks ago Pakistani forces abducted & detained Baloch families from Quetta & Sindh. Now Swiss authorities detain @MehranMarri & his children at Zurich airport on behalf of same forces. Extremely shocking. @ParlCH @NZZ @20min @srfnews @Blickch," an ensuing tweet read.

The president of BRP, Brahumdagh Bugti, has questioned how a neutral country like Switzerland can take sides.

"Really shocked that Mehran Marri was stopped at Zürich airport and banned to enter Switzerland on Islamabad’s request. How a neutral country like Switzerland can do that. Looking forward to meeting with Swiss authorities," he tweeted.

"If any western country including Switzerland have any doubts about our activities then I here today invite all of them to come and be present among us and see what we are doing. This is really what we wanted to let you know the reality," he added.

Images and video: Twitter