The cyclist, a woman believed aged 30, was pronounced dead at the scene [in Dulwich].
Next of kin have been informed. A post-mortem examination will be scheduled in due course.
The 40-year-old driver of the HGV lorry, a white Mercedes, stopped at the scene.
He was subsequently arrested in connection with the collision and taken into custody at a south London police station.
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An eyewitness said: "It was very unnerving. It was just after 12pm and there were suddenly police vans and cars everywhere."