SWR warned passengers to expect reduced services and said the trains that run "are likely to be exceptionally busy".
RMT general secretary Mike Cash said members were striking over "fundamental issues of public safety, security and access".
Whatsapp went down as India celebrated its New Year, prompting thousands to complain they could not send messages to friends and family.
Dubai shunned the more conventional fireworks display in favour of a colourful light show that illuminated the side of the iconic Burj Khalifa skyscraper. Half a tonne of fireworks were set off from Auckland’s Sky Tower as the clock struck midnight at 11am GMT.
We want Londoners to have a great time this New Year but not get so drunk they need our help.“Too often our ambulance crews can spend much of their night caring for people who are vomiting, violent or unconscious after a night out drinking, leaving our staff unable to respond to other emergencies.
Nobody wants to end up in A&E vomiting, injured or unconscious, so please take care of yourselves and your friends.”The Prime Minister used her video address to hail the “good progress” made in Brexit negotiations, but she also made clear that the EU withdrawal was “not the limit of our ambitions”.
Rail strikes are causing disrupting to New Year’s Eve rail services, with passengers facing widespread delays and cancellations.
Members of the Rail, Maritime and Transport (RMT) union on South Western Railway (SWR) and Cross Country have walked out over a long-running battles relating to guards, rosters and Sunday working.
In Australia, 2018 arrived at 1pm GMT with as many as one million people watching Sydney harbour’s world famous fireworks.
London Ambulance Service (LAS) said it took 7,000 emergency calls on 31 December last year, 2,000 more than a typical day.
Extra control room staff, ambulance crews, and support workers have been drafted in to cope with the increase in demand tonight.
10am GMT: Samoa and some Pacific islands are first to see in 2018 as clock strikes midnight 1pm: A million people watch fireworks over Sydney harbour as Australia rings in new year 4pm: Hong Kong shows off 10 minutes of fireworks as midnights also comes in Philippines and China 8pm: UAE calls 2018 'The year of Zayed' with spectacular light display on Dubai skyscraper 11pm: Central Europe waves goodbye to 2017 12am: UK welcomes the new year New Year’s Eve celebrations rolled their way around the world before the UK welcomed 2018 with a massive 12-minute fireworks display over the London Eye.
Thousands also marked Hogmanay in Edinburgh, braving warnings of stormy weather.