CELEBRATIONS marking the 150th anniversary of the Royal Hong Kong Police Force kicked off yesterday with a ceremonial changing of the guard at Government House. Forty officers from the Police Tactical Unit's ''Echo'' company took part in the ceremony in full colonial uniform, as Governor Chris Patten and Police Commissioner Li Kwan-ha looked on. The event was the largest changing of the guard at Government House since the days of former governor Sir David Wilson. The ceremony was quite a contrast for the officers as they had only just come off a four-month tour of duty on Hong Kong's border with China. Chief information officer for the Police Public Relations Branch, Brett Free, said it was extremely rare for any particular police unit to perform two of the most diverse jobs the force has to offer. ''The company was scheduled for guard duty for these three days which just happened to coincide with the 150th birthday celebrations. It's a very special event for these officers.''