Occupy activists have staged a protest over "fat cat pay" on the roof of a central London building.

After meeting in Piccadilly Circus at around 3pm on Wednesday afternoon after the end of the main TUC-organised march to protest against public sector pension reforms, around 200 activists including a steel band made their way through London's theatre district towards an undisclosed location.

Guided by a red flare, the head of the march suddenly entered Panton House, the headquarters of mining giant Xstrata, which occupies the third and fourth floors of the five-storey building. Activists beckoned the crowd forward into the building, which was not guarded.

According to Xstrata's annual report its CEO, Mick Davis, received a pay and free share package worth £17.7m in the last financial year.

More than 40 activists streamed into the building and raced up the stairs on to all floors. Inside, chants were raised against high pay. Police including territorial support group officers arrived within minutes to evict them.

Protesters made their way on to the roof, while below officers blocked the entrance and managed to evacuate the rest of the building. Smoke bombs were also set off which tripped the building's fire alarms.

Currently protesters have been encircled by police on the ground floor of the building and activists appear to be continuing their protest within the containment area.

Another 20 protesters remained on the roof, and police officers were in attendance attempting to get them down.

Many activists left the area soon after the action. Around 45 remained and are currently kettled. One of the activists inside the kettle who only wanted to be known as Morden said: "I'm here because the public sector is getting cut. All the people who are getting hurt by them are the poorest in the country. All the people who don't suffer are the bankers and the rich people."

In a press release Karen Lincoln, a supporter of Occupy London, said: "Mick Davis is a prime example of the greedy 1% lining their own pockets. In this time when the government enforces austerity on the 99%, these executives are profiting. The rest of us are having our pensions cut, health service torn apart and youth centres shut down. We refuse to stand by and let this happen."