The US Geological Survey says the seismic event struck at a depth of 11 kilometres about an hour ago.
South West Police called it a "minor" tremor and said there was no need for people to contact them "unless you have something to report" - such as injuries or damages.
"The walls shook, the lights flashed and the pictures were shaking".
Closer to the epicentre, James Leonard tweeted: "Wow was that an quake in Swansea my house just shook". "I've got quite a lot of glass furniture that was not cheap, so I hope it does not happen again".
Britain's biggest quake in ten years has reportedly been felt from Cornwall to Liverpool.
Tremors were felt across the Bristol Channel into England too.
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There were also reports in north Wales, with some claiming they felt the tremor in Broughton, Flintshire, and Llangollen, Denbighshire.
"For now - it is unexplained - but there is nothing to worry about".
Many took to Twitter to share their experiences as well.
The British Geological Survey is urging anyone living in Cornwall and the south-west to fill in its survery if the felt the quake.
The BGS said such an event only occurs in Britain about every two to three years.