It was not immediately clear if there had been casualties or extensive damage from the quake, which struck at 10:39 am (0609 GMT) around 80 kilometres (50 miles) southeast of Iran's second city in an area called Sepid Sang.
"It was horrible. It made a lot of noise. Everything was shaking," a Mashhad resident told AFP by phone.
Iran lies on a major fault line and sees frequent earthquakes.
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