The CPI(M), whose workers thrashed the two men before handing them over to the police, blamed the RSS for the incident.
Hindu Sena chief Vishnu Gupta said the men, who entered the CPI(M) office posing as journalists, wanted to "express their anger" against the Left party's alleged criticism of the Indian Army in an editorial in its mouthpiece 'People's Democracy'.
The CPI(M), however, has maintained that the editorial criticises Army Chief Bipin Rawat and not the force over his remarks defending the use of a human shield against stone throwers in Kashmir.
Top opposition leaders including Congress chief Sonia Gandhi, her party vice president and son Rahul Gandhi, NCP chief Sharad Pawar, JD(U)'s Sharad Yadav and CPI national secretary D Raja condemned the incident and expressed solidarity with Yechury.
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The incident took place when Yechury was about to enter the central committee hall on the first floor of the CPI(M) office to address the media on the conclusion of the party's two-day politburo meeting here.
Kaul is a driver with a city-based advocate, while Upender works as a cook in canteen at a private hospital, police said.
"They tried to manhandle Comrade Yechury, but we came in their way and he was safe," a party worker present at the spot said.
Speaking to reporters about the incident, Yechury said the two men "would have got adequate responses had they reached close enough to manhandle me".
The Left leader also tweeted, "We will not be cowed down by any attempts of Sangh's goondagardi to silence us. This is a battle for the soul of India, which we will win."
According to police, the matter is being investigated and they have received a complaint from the CPI(M) about the incident.
The police said they were probing the two men's claim that they were associated with the Hindu Sena.
Reacting to the incident, JD(U)'s Yadav said the opposition parties would not allow "efforts (aimed at) shutting their voices".
Raja described the incident as an "attack on the country's communist movement" by right-wing forces, which he alleged, had become aggressive under the NDA regime.
"We now urge the police to take appropriate measures to get to the root of the matter as there could be a sinister design involved," he added.
The Delhi Police had deployed heavy security in view of Kerala Chief Minister Pinarayi Vijayan's presence at the meeting, which began yesterday, anticipating protests by cow vigilante groups.
The southern state of Kerala has also seen many incidents of political violence in the recent past involving CPI(M) and RSS workers.
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