Virtually patting himself for the violence, MNS Secretary Sandeep Deshpande tweeted: "MNS carried out a 'surgical strike' on Bhaiyya Sanjay Nirupam's office. We shall give an apt reply (Eent ka jawab pathhar se milega..)".
Hours later, the Mumbai Police detained Deshpande.
The surprise "confession" by MNS rekindled the ongoing row between the the two parties on the issue of north Indian migrants living and working in Mumbai.
Mumbai Congress President Sanjay Nirupam, himself a north Indian, termed the MNS vandalism as an "act of cowards by impotent and frustrated MNS workers who are routinely getting beaten up by hawkers".
He called upon Maharashtra Chief Minister Devendra Fadnavis to "act fast (against MNS) or a befitting reply will be given for sure".
Taking no chances after the Congress leader's statement, the police beefed up security outside Raj Thackeray's residence near Shivaji Park and MNS offices across the city.
The MNS has revived its anti-migrant agitation after nearly a decade in the wake of the September 29 stampede at Elphinstone Road station. The Congress is fully backing the migrants - with support from Fadnavis and Bollywood actor Nana Patekar among others.
Former Union Minister and senior party leader Gurudas Kamat condemned the attack and demanded that Mumbai Police Commissioner Dattatray Padsalgikar should immediately arrest "the goons".
"Shocking... To what level certain people can stoop is really disturbing. Even worse is to see to what extent they are protected by those in power," Kamat said.
Former minister Milind Deora also tweeted condemning the attack . He said the ruling BJP government turned a blind eye to such violence.
Earlier, around 8 a.m., at least half a dozen MNS activists barged into and vandalized the Congress city headquarters at Azad Maidan when it was being cleaned.
Using sticks and more, they destroyed glass cabins, fittings and furniture in the office before leaving. Though there were no casualties, the office premises were badly damaged.