The mobilisation came as the UN-backed unity government in Tripoli announced the formation of a new military command to take charge of the battle against the jihadists.
The Abu Grein crossroads lies 120 kilometres south of Misrata where the highway along Libya's Mediterranean coast meets the main road south into the desert interior.
It was captured by IS yesterday in an advance from their stronghold in the city of Sirte 140 kilometres to the east.
The head of the Misrata military council, Colonel Ibrahim Bel-Rajab, said he had ordered all brigades under his command to head to Abu Grein without delay, Libya's LANA news agency reported.
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Misrata's two main television channels broadcast appeals to militiamen on leave to return to their barracks.
In a statement, IS said a Tunisian fighter had blown up a vehicle at the crossroads allowing other fighters to advance and take control of it and five other villages in the area, the SITE Intelligence Group reported.
With its port and airport, there are fears the jihadists could use the city as a staging post for attacks on European soil.
The group is estimated to have around 5,000 fighters in Libya, and is trying to attract hundreds more.
Western powers including the United States, Britain and France have openly considered international military intervention in Libya against IS.
They have expressed strong support for the UN-backed unity government which has slowly asserted its authority in the capital Tripoli since the end of March.
The six-member command will be headed by General Bashir Mohammed al-Qadi, it said.