KYIV -- Pro- and antigovernment protesters have squared off in rival demonstrations in the Ukrainian capital, with some people requiring medical treatment after tempers flared at one event.
Police told RFE/RL's Ukrainian Service that "several people" had been injured on the sidelines of the opposition rally after "young people started to throw water bottles and stones." An unidentified aerosol gas was also sprayed during the skirmishes.
Police, dressed in riot gear, eventually intervened.
A police spokeswoman, Olha Bilyk, said an investigation was being conducted to determine what gas was used.
Meanwhile, leaders of the three main opposition factions in the Ukrainian parliament agreed to cooperate on backing a single candidate to oppose incumbent Viktor Yanukovych in presidential elections in 2015.
Meeting in Kyiv ahead of the "Rise, Ukraine!" rally, the leaders of the Svoboda (Liberty), UDAR, and Batkivshchyna (Fatherland) factions stopped short of naming a candidate.
But they agreed that the opposition would have to work together in order to defeat what they called the "anti-Ukrainian, mafia-criminal regime of the so-called Yanukovych Family."
Some opposition members, including former Defense Minister Anatoliy Hrytsenko, have criticized the meeting for failing to strike a clear deal on a single candidate.
Fatherland head Arseniy Yatseniuk said the opposition may support multiple first-round candidates if the current electoral system, which advances two candidates to a second, determining round, remains in place.
But he said the factions were ready to unite behind a unified candidate if the process is changed to a one-round vote.
"If Viktor Yanukovych wants to change the election law and conduct elections in one round," Yatseniuk said, "at the first round he will be opposed by a single candidate from the opposition, who will win at the presidential elections under any circumstances."
WATCH: The ruling Party of Regions rally in the Ukrainian capital on May 18 was a protest against what its members suggest is an effort by political opponents to usher in "nationalism" and "fascism." (RFE/RL's Ukrainian Service)
Both the pro- and antigovernment sides claimed to have drawn tens of thousands of people to their respective demonstrations, which were held amid a heavy police presence.
The pro-government rally by Yanukovych's Party of the Regions was organized to condemn what it suggests is a rise of "neo-fascism." The choice of theme is a swipe at the Svoboda party -- which used to be known as the Social-National Party but rejects associations with National Socialists -- and its opposition allies.
"We are against fascism," a Party of Regions supporter who identified herself as Tetiana told RFE/RL's Ukrainian Service. Then, referring to the generally more pro-EU section of the country, she added: "What is being done in western Ukraine is wrong. I have never seen a meeting for Stalin, but the fact that fascism is trying to rise in Ukraine is very wrong. The deputies from western Ukraine in the parliament are trying to instigate a nationalist mood."
Elsewhere in the capital, a small group of gay-rights activists gathered to protest a lack of legislation preventing discrimination against sexual minorities.
The Verkhovna Rada earlier this month indefinitely postponed the vote on a bill prohibiting job discrimination based on sexual orientation.
One of the activists, Danil, told Reuters that the lack of workplace protection for gays and other sexual minorities was a "shame."
"It’s obvious that total homophobia exists in Ukraine, because according to various statistics, including Ukrainian and European, it’s even sometimes higher than in Russia and Belarus," the activist said.
Additional gatherings supporting the liberalization of marijuana laws and celebrating Europe Day were also held in Kyiv.
In the southern city of Simferopol, commemorations were held to mark the 69th anniversary of the forced deportation of Crimean Tatars ordered by Soviet leader Josef Stalin at the end of World War II.
With additional reporting by Reuters