Amid mounting signs of deadlock, Yvo de Boer urged delegates from 193 countries to overcome their differences and produce a deal.
"I still believe it's possible to reach a real success," he said on Wednesday. "The next 24 hours are absolutely crucial and need to be used productively.''
On Thursday environment ministers are set to meet for final rounds of negotiations, aiming to salvage a climate pact from half-finished draft texts and overcome long-running rifts between rich and poor nations over how to split the costs of fighting climate change.
The meetings come as dozens of heads of state descend on Copenhagen, hoping to add their weight in the final days of the talks.
About 120 leaders are expected to attend the summit, with security being tightened in the Danish capital following days of escalating protest.
On Wednesday protesters angry at the lack of progress at the climate change talks in Copenhagen clashed with police in the Danish capital.
Police fired pepper spray and beat protesters with batons, making more than 200 arrests.
Impromptu protests also took place inside the conference hall.
Among the world leaders set to address the talks on Thursday are the presidents of Iran, France and Brazil.
Barack Obama, the US president, is due to arrive on Friday, as is the Chinese premier Wen Jiabao.
Speaking at the White House, Obama spokesman Robert Gibbs said the appearance of leaders from around the globe, including Obama, creates the opportunity for a "breakthrough to happen."
But delegates who have attended several days of gruelling and often tense negotiations, said the slow pace of the talks signalled the summit could be headed for failure.
"We may not get there on the substance, it is quite possible we'll fail on the substance, but at least let's give it a try," said Ed Miliband, Britain's energy and climate minister.
"At the moment the problem is we're not giving it a try."
Denmark, whose prime minister is now chairing the summit, said it was trying to simplify several complex draft negotiating texts to help the leaders attending a final high-level summit on Thursday and Friday agree on a deal.
However delegates from developing nations have rejected Danish proposals to select small negotiating groups to storm through the draft texts, saying the process had to be fully inclusive.
Among the key sticking points is a long-running rift between developed and developing nations over who should cut emissions, how deep the cuts should be, and how much assistance the rich world should provide to poor countries.
The US and China, the world's top carbon emitters, have also been stuck in a dispute over how they will prove they are sticking to emission-curbing plans.