"The debate did have an impact. We did see a spike," said Jenny Watson, the head of the commission.
A good performance in the debate by Nick Clegg, the opposition Liberal Democrat leader, has resulted in a dramatic surge in popularity for his party in the latest opinion polls.
The Liberal Democrats now hope to win significant numbers of voters away from the ruling Labour party and main opposition Conservatives.
That makes it more likely that the election could produce a "hung parliament" in which no single party has an overall majority.
Such an outcome could force two of the parties to make an alliance, but it is unclear who would team up with whom.
The electoral commission said on Wednesday that as well as the debate, a partnership with the Facebook social networking website over the weekend had contributed to the spike in interest in voting.
Over the weekend, which was the last before the deadline to register, people logging onto Facebook from Britain were presented with a message asking them whether they had registered and directing them to the commission's website.
"The Facebook link really helped us," said Watson.
Another factor was that the deadline to register was on Tuesday, which could have prompted a last-minute decision to register from many voters, the commission said.