Police with dogs fired tear gas and arrested around 100 marchers near the Bella Center amid growing unease brewed over the snail-like progress -- and on some issues, utter deadlock -- at the marathon negotiations.
Around 1,500 climate demonstrators tried to march on the closely guarded conference centre where top leaders gathered.
Police set up posts on bridges and roads leading to the venue, stopping cars and buses for checks. Police were especially on alert in the working class area of Noerrebro, where many foreign activists are staying.
Some of the world's leaders, arriving ahead of Friday's climax when some 120 chiefs will be in attendance, began to portray the negotiations in a sombre light.
British Prime Minister Gordon Brown acknowledged a deal would be "very difficult," while his Australian counterpart Kevin Rudd said there was "no guarantee" of accord.
UN Secretary-General Ban Ki-moon said the talks were a "defining moment in history" that could not be ignored by any country.
But he admitted an agreement on financial aid for developing countries may have to wait until a later date.
If all goes well, Friday's summit will conclude with a post-2012 strategy for shrinking climate change from mortal peril to a manageable threat.
It would set down the outlines of an accord on curbing carbon emissions that cause global warming and craft a mechanism to provide billions of dollars for poorer countries in the firing line of climate change.
Further negotiations would unfold in 2010 for agreeing on details.
Scientists warn that many millions of people face going hungry, losing their homes and access to water within the next decade if nothing is done to stem the rise in greenhouse gas emissions.
But nine days of talks among lower-level officials and informal negotiations among groups of ministers have failed to produce a breakthrough on any of the key -- and tightly intertwined -- issues.
"I am just a little bit worried, because there is still so much work to do," French climate ambassador Brice Lalonde told AFP.
"I fear there could be another blow-up, just when we are struggling to get an agreement."
Some of the bitterest wrangling has been between the world's two biggest carbon emitters, China and the United States, who declared on Tuesday they would not shift on their emissions pledges, the thorniest problem of all.
US President Barack Obama has offered to cut US carbon emissions by 17 percent by 2020 over a 2005 benchmark, a figure that aligns with legislation put before the US Congress.
It amounts to a reduction of around four percent compared with the more widely used reference year of 1990. The European Union has pledged to cut its emissions by 20 percent on 1990 levels by 2020.
The 12-day confab in a giant convention centre on the outskirts of Copenhagen has been mired with organisational problems with the number of people accredited outstripping capacity by around 30,000.
Thousands of activists from non-governmental organisations were being forced to leave the venue on Wednesday to accommodate the influx of VIPs, further fuelling anger among those who already feel their voices are not being heard.
Many of those now excluded joined a march by around 1,500 activists towards the summit venue, led by non-governmental groups in Denmark and developing nations.