"Nath and Jaki Jakir were beaten up by a mob of more than 200 people at a hotel in the town where they were staying late last night", Superintendent of Police Pradip Pujari said.
The mob was angry at the second marriage of Nath to Jakir last month, police sources said.
Nath, who is pregnant, and Jakir were severely injured and profusely bleeding before they were whisked away by police immediately after the incident, the sources said.
The duo, after medical treatment, is being escorted to the state capital Guwahati, Pujari said.
Nath, representing Borkhola assembly constituency in Barak Valley, had created a furore by announcing her marriage with Jakir without divorcing her first husband Rakesh Singh with whom she has a two-year-old daughter.
Singh had filed an FIR that his wife was missing last month.
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This was the first instance of a woman legislator being beaten up in public.
Rumi Nath was first elected to the Assembly in 2006 on a BJP ticket from Borkhola constituency and later defected to Congress and won the seat a second time in the 2011 elections.
The MLA along with her second husband were treated by a team of four specialist doctors of the Guwahati Medical College Hospital and were reported to be out of danger.
The injured MLA in tears said it was an "unfortunate incident" and said the culprits who had "outraged the modesty of a woman", should be brought to book.
Nath and her husband were being guarded in their MLA quarter in the Dispur capital complex by a strong contingent of police and CRPF forces.