The panic was apparently sparked by movements of government troops in the area, which has remained volatile since the end of a five-year civil war in 2002 that sucked in six Central African nations.
Congo has recently deployed additional forces in its eastern provinces in what it says is an operation to quell fighting by a Rwanda-backed rebel group.
In Ngungu, 40 miles northwest of the eastern city of Goma, about 20,000 people from a dozen villages in Kalehe district who fled their homes over the past two weeks were combing rain-soaked hillsides on Saturday in search of shelter, said Gemma Swart, a media officer with the British aid group Oxfam on Saturday.
Some were trying to build makeshift huts out of bamboo, but few had roofs and most people were simply sitting outside.
"Conditions are very difficult. It is very cold," Swart said. "People have been trying to set up shelters but there is an urgent need for plastic sheeting, blankets and sanitation."
There were no reports of attacks in Kalehe, Swart said, adding that the displaced apparently feared they could be attacked by the military or other armed groups.
Swart was among several Oxfam staff who travelled to the area to assess the plight of the displaced. She said the aid agency would help build latrines and set up points to distribute potable water in Ngungu next week.
Last week, pro-government militia fighters battled former rebels for control of Walikale, further to the west, sending 15,000 other people fleeing.
In June, thousands of renegade Congolese troops seized Bukavu, a lakeside town to the south, holding it for a week before loyalist forces repelled them.
Nearly 11,000 UN peacekeepers are deployed in the country to help secure the peace, but they have been unable to stop the fighting.