The first in a series of planned strikes by Cardiff Bus drivers is under way in the city.
More than 500 Unite members are taking action after rejecting a 5% pay deal.
Cardiff Bus is running a limited service and apologised to travellers but said it was able to run more routes than it anticipated.
Ten strike days have been planned, coinciding with a forthcoming rugby international and World Cup matches.
About 250 workers joined a march and rally in Cardiff city centre on Wednesday organised by the Unite union.
What is it about?
A 3% pay rise, back-dated to April, followed by a 2% increase in 2016, has been accepted by some Unison members.
But it has been rejected by Unite members, who make up 70% of the workforce. They voted 83% in favour of industrial action in a ballot.
Cardiff Bus says its drivers earn on average at least 15% more than its closest local competitor and with dwindling passenger numbers it cannot afford to pay any more.
Unite accuses the company of "intransigence" over negotiations and said it has been working flat out for 10 months to find a solution.
What will the action mean?
Cardiff Bus normally carries 100,000 passengers a day on 3,000 journeys; roads are likely to be much busier with city schools back and the end of most people's holidays.
Special timetables will include services to outer suburbs such as Ely, Llanderyn, St Mellons, Thornhill - and the University Hospital of Wales
Newport Bus will accept Cardiff Bus tickets on strike days for the service between the two cities.
Passengers are advised to check the Cardiff Bus website, Facebook page and on Twitter @Cardiffbus
Strike days have been organised to coincide with rugby internationals at the Millennium Stadium - including Rugby World Cup matches. These could put extra pressure on the roads.
One passenger said the lack of information on the strike was 'ridiculous'
Cynthia Ogbonna, managing director of Cardiff Bus, said they were able to run a limited service, while most of its maintenance team worked as normal.
"We were able to put on extra routes to those that we had anticipated due to more drivers being available," she said.
"We have been doing our upmost to keep Cardiff moving and we would like to thank our customers for their support and patience during this time.
"We are continuing to work hard to resolve this matter as soon as possible."
Last-minute talks at conciliation service Acas failed on Tuesday to avert the strike.
Alan McCarthy, Unite regional organiser, said: "This decision to take industrial action is a last resort which was not taken lightly, however over 83% of our members are taking the action which reflects the strength of feeling amongst our members."
