The vigil late on Wednesday was called to "bury the regime" of President Faure Gnassingbe, whose March 4 re-election triggered opposition charges of voter fraud, the opposition said.
Police used tear gas after demonstrators detained a plainclothes policeman, witnesses said. At least 30 people were injured in ensuing scuffles, opposition officials said without giving further details.
There was no official confirmation on the number of injured.
In a statement, the ministry of security and civil protection said opposition protestors had blocked main roads and disrupted traffic.
"All forms of protest related to the results of the March 4 presidential poll are strictly banned," it said.
The opposition has organised several protests over the outcome of the election.