Of the infections, 964 were domestic transmissions, including 236 in the capital Bangkok, the epicentre of an outbreak that has spread to most of Thailand's 77 provinces.
The spike comes ahead of major national Songkran holidays, known for big street water fights that authorities have now banned for a second year due to the pandemic.
Authorities have urged people to avoid unnecessary travel and reduce gatherings to help limit the outbreak, which includes the highly transmissible B.1.1.7 variant first identified in Britain.
"Right now, the situation is worrying. But we can manage it if everyone helps," senior health official Sopon Iamsirithaworn told a briefing.
At least 38 provinces have required visitors from risk areas to undertake a 14-day quarantine. Nightspots such as pubs and karaoke bars in Bangkok and 40 provinces will be closed until April 23.
The managers of two entertainment venues in Bangkok, where many infections were reported, had been sentenced to two months in jail for Covid-19 violations, police said on Sunday.
Thailand has reported 3,661 domestic infections so far this month, including 1,294 cases in Bangkok, the highest amid the new outbreak. That brings total to 32,625, with 97 deaths.
AFP reported that thousands of Thais traditionally return to their home provinces during the Songkran holiday period, which officially starts Tuesday.
Prime Minister Prayut Chan-O-Cha encouraged people to stay home during the holidays.
"Please don't hold activities which will draw many people together including water fights or foam parties," he said in a podcast on Saturday.
The national government has not brought in a travel ban but, as of noon Sunday, 38 out of 77 provinces had announced their own entry restrictions.
They are asking people from Bangkok, Nakhon Pathom, Pathum Thani, Nonthaburi and Samut Prakan to self-isolate for 10 to 14 days.
On Friday authorities ordered the closure of all entertainment venues in 41 provinces for two weeks.
Thai police announced Sunday they were taking legal action against two Bangkok night spots in Thonglor for breaching pandemic restrictions and contributing to the outbreak.
"We charged managers of both clubs and a court ruled two months jail for both," police major general Sophon Sarabhat told reporters.