Around 170,000 workers in Wales have gone on strike as part of UK-wide industrial action over pension changes.
More than 90% of pupils across the principality are missing classes today as part of the public service disruption.
Waste collection, libraries and other services were affected, while non-urgent hospital operations have been rescheduled.
All Cardiff Bus services have been cancelled and funeral services up and down the country have had to be rescheduled.
Pickets were set up early this morning around Wales, including at the Driver and Vehicle Licensing Agency (DVLA) in Swansea.
Some staff were ignoring the picket at the DVLA, although union officials said it was a fraction of the normal number of people working.
Across Wales, more than 1,500 out of 1,776 schools have closed.
In some areas, including Cardiff, Blaenau Gwent, Caerphilly and Carmarthenshire, all schools are closed.
The NUT said the decision to take strike action was not made lightly.
Owen Hathway of NUT Wales said: "We anticipate the vast majority of schools in Wales will be closed as a result of the action taken on Wednesday by the NUT and other unions.
"No teacher who has been forced by the government to lose pay and go out on strike has done so lightly.
"This is not action we desire and even at this stage we would like the Government to say they want to talk."
Council services including libraries, refuse collections, leisure centres, museums and car parking were all affected.
In some areas, councils have made local agreements with unions to provide essential services such as home and respite care.
Blaenau Gwent Council is operating an emergency and "life and limb" service only, as it estimated it would have fewer than 2% of 4,000 staff in work.
And health boards across Wales cancelled hospital appointments and most planned operations, although emergency and essential care such as chemotherapy and dialysis appointments will go ahead.
In most areas, routine appointments and non-urgent elective surgery will be re-scheduled.
But emergency and essential healthcare will continue, due to agreements struck with unions.
As well as urgent surgery, cancer care and dialysis treatment should be provided as usual.
Libraries and leisure centres were disrupted, while some councils - such as Swansea - postponed burials and cremations.
Also in Swansea, the council confirmed all its public toilets were closed today.
In Rhondda Cynon Taf all libraries, leisure centres and day centres were closed.
Cardiff Bus services have been cancelled after the company was told workers from two unions would be on strike.
"We understand other bus companies, and the train companies, will be operating as normal so some alternative services will be available but these may be very crowded," said a spokesman.
Rush-hour traffic in the Welsh capital was much busier than usual, with a number of delays reported - exacerbated by the closure of the Butetown Tunnel.
Picket lines formed outside County Hall in Cardiff and the Welsh Assembly's administrative offices Ty Hywel.
A march through the Welsh capital was scheduled for midday, culminating in a rally at the Swalec Stadium, Sophia Gardens.