Vaishali superintendent of police Rakesh Kumar said the crime took place at Satihara village, close to the district headquarters.
When the villagers found the woman's body hanging from the tree this morning, they blocked the busy NH 77 demanding immediate arrest of the perpetrators.
The body, which did not bear any superficial injury mark, was brought down and sent for post-mortem, he said.
A dog squad was also sent to the village to track the accused, Kumar added.