Hampshire Police have closed off section of East Park Terrace in the centre of Southampton this after afternoon.
A spokesperson from Hampshire police said "we are currently deal with a serious road traffic collision involving a cyclist and a lorry."
The road between New Road and the Jury's Inn roundabout is closed in both directions.
New Road is closed for traffic heading out of town, between Above Bar Street and East Park Terrace.
Please could motorists avoid the area as we deal with this incident.
Police anticipate that the roads will remain closed into the rush hour and we thank the public for their patience as we deal with this incident.
The cyclist has been taken to Southampton General Hospital with life-threatening injuries.