Police in the Democratic Republic of Congo have used tear gas and batons to disperse hundreds of opposition supporters in the capital, Kinshasa.
Tshisekedi's supporters boycotted the initial voter registration
They want to register for the country's first democratic elections in 40 years set for 18 June, after boycotting the initial voter registration process.
They are angered that President Joseph Kabila has approved the electoral calendar allowing nominations.
The polls are to end a power-sharing period after a five-year civil war.
The protesters in Kinshasa were among thousands of supporters of the veteran opposition leader, Etienne Tshisekedi, who held demonstrations across DR Congo.
Many chanted the party slogan, "Viva, viva," and carried red, white and blue flags, Reuters news agency reports.
Mr Tshisekedi, who was sidelined by the former warlords who joined the power-sharing government, had initially boycotted the electoral process.
The UN mission in DR Congo (Monuc) condemned the police action as heavy-handed.
"On first account there seems to have been a disproportionate use of force, and if this is the case, we strongly condemn it," Monuc spokesman Kemal Saiki told the AP news agency.
"The people have a constitutional right to express their opinion and demonstrate."
The BBC's Arnaud Zajtman in Kinshasa says the publication of the electoral law and timetable coincides with tension among other political groups.
Joseph Kabila is favourite to win the presidency
The former Rwandan-supported rebel group, the Congolese Rally for Democracy, has threatened to pull out of the government if one of its strongholds is not established as a constituency before the polls.
Meanwhile, the former Ugandan-supported rebel group, the Movement for the Liberation of Congo, has dismissed the chairman of the national assembly.
But President Kabila, the favourite to win presidential elections, has said that the time for power-sharing among former enemies is now over.
He came to power after his father, Laurent - who overthrew long-time ruler Mobutu Sese Seko - was assassinated in 2001.
According to the peace deal that was signed by the former warring factions three years ago, the elected government and parliament were to be sworn in by 30 June.
But the newly published timetable says that the results of the first round of the presidential elections will only be revealed on 14 July.
Some 17,000 UN peacekeepers are in the country to oversee the elections, but armed groups continue to rampage in parts of the east.