Website crash detection site DownDetector reported the main outages in Manchester, Birmingham and London.
One user said: "Twitter's working on my phone but not on my computer. Kinda perplexed
Down Detector, which monitors service issues on various websites, said user reports pointed to problems with Twitter.
Of those in the UK reporting issues, 91% said they were unable to access the website. 6% raised concerns about the server connection and 3% said they were having problems with the app.
Twitter has not yet addressed what is causing the outage with the website. But the phone apps do not seem to be having the same issue.