At least 65 people have died after a powerful earthquake struck the southern New Zealand city of Christchurch, collapsing buildings, burying vehicles under debris and sending rescuers scrambling to help people trapped under rubble. The 6.3-magnitude quake struck the country's second largest city on a busy weekday afternoon. The mayor of Christchurch, Bob Parker, has declared a state of emergency and ordered people to evacuate the city centre. "Make no mistake this is going to be a very black day for this shaken city," he said. Power and water was cut and hundreds of dazed, screaming and crying residents wandered through the streets as sirens blared throughout Christchurch in the aftermath of the quake, which was centred three miles from the city. The US Geological Survey said the tremor occurred at a depth of 2.5 miles. The multi-storey Pyne Gould Guinness Building, housing more than 200 workers, has collapsed with an unknown number of people trapped inside. Television pictures showed rescuers, many of them office workers, dragging severely injured people from the rubble. Elsewhere, police said debris rained down on two buses, crushing them, while emergency workers were moving to rescue survivors trapped in other partially collapsed buildings across the city. New Zealand's TV3 said 24 people were trapped on the 17th floor of the 19-storey Forsyth Barr office building, near the cathedral. The building was intact but a stairwell had collapsed, it said. Christchurch hospital had to deal with many injured residents.