More than 200 athletes and officials from 19 Asian countries have gathered in Guwahati, the state's main city, to participate in a grand prix organised by the Asian Athletics Association.
The separatist United Liberation Front of Asom (ULFA) - fighting for independence for the oil- and tea-rich state of 26 million people - is suspected to be behind the blast.
ULFA had threatened to disrupt the National Games, India's leading sporting event, held in the state in February.
Though the outfit has issued no specific threat against the Asian event, it says such events are organised by New Delhi to reinforce its claim that Assam is an integral part of India.
Separatist violence has killed more than 20,000 people in Assam since 1979.