Protesters crossed the first security barriers of the venue and were then prevented from getting further in, the UN told the BBC.
A security guard told the Reuters news agency he was hit in the head by a drum thrown by a protester.
The talks are hosting delegates from almost 200 countries, including senior politicians from across the world.
The talks are being held in Belém on the edge of the Amazon rainforest, which is home to dozens of indigenous groups.
Many are vocal critics of the environmental damage caused by climate change and deforestation.
