Police guard the centre as relatives of those killed gather
Police say he then killed himself and his family after a manhunt following the attack in Nong Bua Lamphu province.
Children and adults are among the casualties at the nursery - police say the attacker mostly stabbed his victims before fleeing the scene.
The former officer, aged 34, was sacked in June for drug use, police said.
It is not clear if there was a motive for the attack.
At least 23 children were among the dead in the mass killing in the town of Utthai Sawan. Some victims aged as young as two were attacked as they slept.
A dozen people who were injured have been taken to Nong Bua Lamphu district hospital.
Headteacher Nanticha Panchum said the attacker's son attended the centre but had not been there for a month.
The man used to drop his child off, and was always polite and chatty, she said.
Ms Panchum said there were usually more than 90 children at the school, but just over 20 were present on Thursday due to bad weather and a school bus breakdown.
"The shooter came in around lunchtime and shot four or five officials at the childcare centre first," a local official, Jidapa Boonsom, who was working nearby, told Reuters news agency. One of them was a teacher who was eight months pregnant.
"At first people thought it was fireworks," she said, adding that the man then forced entry to a locked room where children were sleeping.
Videos shared on social media appear to show the tears and distress of parents and relatives of those killed, as they gathered at a shelter outside the centre.
Police officers arriving after the attack were confronted with horrific scenes, the bodies of adults and children, some of them very young, lying inside and outside the building.
"After inspecting the crime scene, we found that the perpetrator tried to break in and he mainly used a knife to commit the crime by killing a number of small children," said Police Chief Damrongsak Kittiprapat.
"Then he got out and started killing anyone he met along the way with a gun or the knife until he got home. We surrounded the house and then found that he committed suicide in his home."
Thai Prime Minister Prayuth Chan-ocha described the incident as "a shocking event".
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The shooter appears to have travelled from the nursery in Uthai Sawan to his home in the nearby village of Ban Tha Uthai, where he killed himself and his family
Police named the attacker as Panya Kamrab, a local man who had been a police sergeant before he was suspended for drug use in January, and then dismissed in June. He is understood to have appeared in court on Thursday on charges related to the use and possible sale of methamphetamine. The verdict was scheduled to be delivered on Friday.
Armed with a shotgun, a pistol and a knife, he stormed the nursery at about 12:30 local time (05:30 GMT).
The details of what followed are still emerging, but after the killing spree the attacker fled the scene in a white four-door Toyota pick-up truck with Bangkok registration plates, according to police, who launched a search for him and warned locals to keep indoors for their own safety.
Eyewitnesses were quoted saying the attacker had driven into bystanders and opened fire at some of them, injuring several people as he made his escape.
Police say Kamrab returned home, killing his wife and son before taking his own life.
Mass shootings in Thailand are rare, although gun ownership rates are relatively high for the region. Illegal weapons are also common in the south-east Asian country, according to the Reuters news agency.
The nursery attack comes less than a month after an army officer shot dead two of his colleagues at a base in Bangkok.
In 2020 a soldier killed 29 people and injured dozens more in the city of Nakhon Ratchasima.
-- Courtesy of BBC News