The flag, which read “Iran belongs to its people” and “Freedom for Iran,” was attached to a backpack and dropped from the 100-foot (34-meter) tower ledge as crowds gathered below, according to witnesses.
Videos posted on social media showed the man holding the dog, believed to be a Pomeranian, while displaying the banner.
British Transport Police (BTP) said officers were called at around 8 a.m. after reports of “a person in a precarious position.”
“The incident is ongoing and officers are in attendance alongside other emergency services, working to bring the incident to a safe conclusion,” a BTP spokesperson said before noon.
The London Fire Brigade and London Ambulance Service were also deployed, with fire crews setting up an aerial ladder platform as a precaution. A police cordon was put in place around the busy transport hub in central London.
The protest comes amid heightened tensions in the Iranian diaspora in Britain. In June, seven Iranian nationals were charged with grievous bodily harm with intent after a fight broke out outside Iran’s embassy in London, in what police described as a clash between activists opposed to and supportive of the Islamic Republic.
By late morning on Tuesday, the man remained on the tower as authorities negotiated with him. Passengers continued to use the station, one of London’s busiest, although parts of the concourse near the tower were restricted.
There was no immediate comment from the Iranian embassy in London.