Troops in the Democratic Republic of Congo blocked entry to a stadium where the main opposition leader planned to inaugurate himself as president.
Etienne Tshisekedi has rejected the official victory of incumbent President Joseph Kabila in November's elections.
A BBC reporter says police used stun grenades to disperse people attempting to get to the stadium in the capital.
So Mr Tshisekedi's conducted the unofficial inauguration at his home residence instead.
President Kabila was sworn in for a second term on Tuesday after being declared the official winner with 49% of the vote, compared to Mr Tshisekedi's 32%.
Western observers denounced the presidential results as seriously flawed, but the election commission - backed by the African Union - hailed the polls a success.
The US-based campaign group Human Rights Watch (HRW) says police have killed at least 24 people since the disputed polls.
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