Police were questioning the Spanish-registered driver of the lorry following the accident on Saturday evening in Nieuw-Beijerland, a village 30 kilometres (18 miles) south of Rotterdam.
The victims were three women aged 28, 32 and 75, and three men aged 41, 50 and 62, all from the local area, police said. One of the injured was in a critical condition in hospital.
Dutch King Willem-Alexander said he was “shocked” and Prime Minister Mark Rutte offered condolences over the “terrible” accident.
Police had first confirmed at least two dead after the vehicle drove into the crowded community barbecue, which Dutch news reports said was being hosted by an ice-skating club.
“We now have six people killed and seven injured, one seriously after yesterday’s accident,” police spokeswoman Mirjam Boers told AFP.
“We are still investigating what exactly happened,” she said.
Witnesses said the truck stopped for a moment at a T-junction on the narrow Zuidzijdsedijk dike, before taking off and ploughing into the revellers, according to local broadcaster Rijnmond.
Photographs from the scene taken immediately after the accident identified the truck as belonging to the El Mosca company, based in Spain. They showed scattered chairs lying on the ground and bunting hanging from trees.
– ‘Ripped from their lives’ –
Local mayor Charlie Aptroot said the accident had devastated the tight-knit community.
“Six residents have literally been ripped from their lives. One of the victims was heavily pregnant,” he said, adding that children were among the injured.
Three of the dead — a mother, son and daughter-in-law who was eight months pregnant — came from the same family, Rijnmond reported, quoting a local church announcement.
Residents were laying flowers at the location of the accident on Sunday, an AFP journalist at the site said. A police car stood guard at the scene.
“When we got here it was really terrible. There were a lot of police, ambulances, fire trucks, they were all here,” local resident Bob van den Burg, 20, who went to the scene after the accident on Saturday, told AFP.
“Everybody is affected and mourns with the people here. Nothing ever happens here, there are almost no thefts or something terrible, and suddenly something like this happens.”
– ‘Unimaginable sadness’ –
Police spokeswoman Boers confirmed the 46-year-old driver from Spain had been arrested “and was not under the influence of alcohol” at the time of the accident.
“He was arrested for causing a fatal accident and serious bodily injury. His role in the incident is under investigation,” police said in a statement.
Investigators had taken blood samples to see if he was on any medication and were also checking whether he was using his phone at the time of the accident, Dutch prosecutors said.
King Willem-Alexander and Queen Maxima said they were “shocked” by the tragedy.
“We are shocked and moved by the terrible accident in Nieuw-Beijerland last night in which so many people were killed. An unimaginable sadness within this close-knit community,” they said in an official message.
Premier Rutte also sent condolences.
“Terrible news… my thoughts are with the victims and relatives of this terrible drama. I wish them a lot of strength,” Rutte said on Twitter.
People from the surrounding region travelled to the village to offer their support.
“We live in another town close by and we all heard the sirens,” said Jolanda Koster, 61.
“Of course you mourn with these people. Now we are here to lay some flowers to show we are thinking of the relatives and all the other people that have been affected.”