Three women and three men have died in a road accident involving four vehicles on the A23 near Brighton.
It is believed a car crashed through the central reservation
Three other people - a two-year-old child, a woman in her 20s, and a man in his 30s, were seriously injured.
Three more people were taken to hospital with minor injuries but have since been discharged.
The A23 was closed in both directions, causing serious tailbacks, but Sussex Police said it had been re-opened at about 0745 BST on Monday.
Inspector Steve Higgs of Sussex Police said the driver of a black BMW travelling south appeared to have lost control of his car, which crashed through the central reservation onto the other side of the road.
He said it struck several vehicles on the opposite side of the road.
The road was closed in both directions throughout Sunday night
A Sussex Police spokesman described the scene of the crash, between Patcham and Pyecombe, as "one of some carnage".
The two-year-old child hurt in the crash was admitted to the Royal Sussex County Hospital in Brighton with "very serious injuries".
The man and woman detained overnight were being treated at the Princess Royal Hospital in Haywards Heath.
A hospital spokesman said on Monday morning the child and one of the adults were in a critical condition, while the other adult was in a serious condition.
"This is a very serious and rather nasty accident," a spokeswoman for Sussex Ambulance told BBC News Online.
A senior paramedic at the scene had said it was "one of the most horrific incidents he had attended", she added.
The ambulance service is offering counselling to witnesses, some of whom had tried to resuscitate victims, the spokeswoman said.
She said: "We are concerned that they may have been affected by what they have seen."
Police want any witnesses to the crash to contact them.
Insp Higgs said the road had been kept closed to ensure the "painstaking" process of gathering evidence could be carried out.
He said: "This is without doubt one of the most serious incidents of its kind to occur in Sussex in the last few years.
"We are doing everything we can to ensure that bereaved relatives and those involved have answers to the many questions which will arise over the coming days and weeks."