TEHRAN, July 31 (Reuters) - An earthquake with a magnitude of 5.7 hit northeastern Iran, injuring around 170 people, state radio reported on Saturday.

The tremor hit the city of Torbat-e Heydariyeh, about 700 km (435 miles) east of Tehran, at 6:20 p.m. (1350 GMT) on Friday, and had its epicentre around 7 km outside the city.

Many people spent the night outdoors in the city's parks fearing aftershocks.

Of the 170 injured, 22 needed hospital treatment while the others were treated on site. No deaths were reported.

Earthquakes are common in Iran. In 2003, about 30,000 people were killed in a quake that devastated the southeastern city of Bam.