About a kilometre west of Tiananmen Square and the compound housing China's top leaders in central Beijing stands a large unmarked building. No sign hangs at the entrance to indicate the business conducted inside. The occupant's phone number is unlisted; calls from the building do not display an incoming number identifying their origin, just a string of zeros.

As the Communist party on Thursday marks 60 years in power, however, the occupants of the office complex will be quietly celebrating as well. Little known even within China, the body based there and known as Zhongzubu – the Central Organisation Department – has emerged from the country's economic upheaval of the past three decades as indispensable to the party's hold on power.

China's embrace of the market since the late 1970s has driven a surge in economic growth and a social revolution. Chinese citizens are in many respects freer and richer than they ever have been under communism, able to work where they want, travel overseas and buy homes and cars. But while easing controls over aspects of the economy and society, the party has worked to ensure it maintains its grip on other levers of power.