"I am concerned by the restrictions imposed on peaceful assembly and arrests of those who seek to express their political views, as well as by the targeting of journalists and the confiscation of their materials," said UN Mission in the Congo (MONUSCO) head Maman Sidikou.
On Monday the United Nations documented more than 120 arrests or detentions in Kinshasa, Goma and other towns, including local and foreign journalists, he added.
Police and opposition protesters clashed in Goma on Monday as demonstrators called for elections.
Political tension is rising steadily in DR Congo as the pressure mounts on Kabila, whose mandate expired on December 20 last year, to organise elections.
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Under an agreement reached at the end of last year, Kabila was to hold them by the end of 2017, under the aegis of the influential Roman Catholic church.
On July 7, however, CENI announced it would not be possible to organise elections in the vast and troubled country by year end.
He also stressed the need for all political stakeholders "to refrain from any statement or action that could heighten tension and further polarise the political landscape".
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