Mr Miliband and David Cameron both faced questions from Jeremy Paxman and audience members in the show, which was broadcast on Sky News and Channel 4.
A spokesman for communications watchdog Ofcom said it had received 131 complaints by late morning - all of which were about perceived bias against the Labour leader.
An Ofcom spokesman said: “We are assessing the complaints before deciding whether or not to investigate.”
A Channel 4 spokeswoman said it had received around 300 complaints, but was unable to say specifically what they were about.
The two men faced an aggressive grilling over their personalities and policies from Paxman but did not debate one another directly, after Mr Cameron refused to take part in a head-to-head showdown with the man vying to replace him in 10 Downing Street.
At one point, the former Newsnight presenter told Mr Miliband some of his MPs viewed him as a “liability”, while he was seen as a “north London geek” by some voters, who thought “what a shame it’s not his brother”.
He was also heard asking him “Are you OK, Ed? Are you all right?” after the on-screen grilling.
The live 90-minute show, called Cameron & Miliband Live: The Battle For Number 10 pulled in an average audience of 2.6 million viewers on Channel 4 and a further 322,000 on Sky News.
Channel 4 boss Jay Hunt described the show as “vintage Paxman”.