Moscow , Russia ( CNN ) -- Thousands of opposition demonstrators marched in front of the parliament building in Kiev , Ukraine , Saturday , protesting a deal reached earlier this week to extend Russia 's military presence in the former Soviet Republic , national news media reported . Parliamentary opposition leader Yulia Tymoshenko -- the former prime minister who lost to Viktor Yanukovych in the presidential election run-off in February -- told protesters Saturday that the ratification of the treaty must be prevented at all costs . She claimed that Yanukovych is `` selling out '' Ukraine , has `` openly embarked on the path of destruction of [ Ukraine 's ] national interests , and has actually begun the process of eliminating the state 's sovereignty , '' according to a transcript of the speech on her website . After the deal was signed Wednesday by Yanukovych and Russian President Dmitry Medvedev , Tymoshenko said it violated part of the Ukrainian Constitution , which forbids the country from hosting foreign military bases after 2017 . Saturday , protesters reportedly adopted a resolution calling the agreement an `` unprecedented act of national treason and disgrace , '' and calling on all opposition groups to unite against it . According to Tymoshenko 's website , some 10,000 people gathered at the rally . But Ukrainian national news agency UNIAN estimated the number of protesters at 5,000 . The deal extends Russia 's lease of a major naval base in the Black Sea port of Sevastopol , Ukraine , for an additional 25 years , in exchange for a 30 percent cut in the price of natural gas that Russia sells to Ukraine . The agreement may bring an end to years of disputes over natural gas prices , which culminated in Russia turning off the pipeline to Ukraine . The dispute affected not only Ukrainians , but many Europeans who depend on Russian gas pumped through Ukraine . The two countries had been at odds ever since the `` Orange Revolution '' swept Yanukovych 's fiercely anti-Russian predecessor Viktor Yushchenko to power in 2005 . Throughout his time in office , Yushchenko repeatedly threatened to expel Russia 's Black Sea Fleet from Sevastopol . The Russian military lease there was scheduled to expire in 2017 . `` The prolongation of the Black Sea Fleet 's presence in Sevastopol is essential to Russia , '' Yanukovych said Wednesday . `` We understand that the Black Sea Fleet will be one of the guarantors of security on the Black Sea . '' The Kremlin-friendly Yanukovych , who hails from predominantly Russian-speaking eastern Ukraine , trounced Yushchenko in national elections last January . The Russian president said the new deal added a `` concrete and pragmatic dimension '' to centuries of relations between Ukrainians and Russians . Opposition groups in Ukraine , however , were quick to denounce the agreement . Yuschenko 's `` Our Ukraine '' party said the treaty would lead to the `` Russification '' of Ukraine . Opposition activists decided Saturday to stage another protest in front of parliament April 27 , when the deal will be put to a ratification vote . CNN 's Ivan Watson and Maxim Tkachenko contributed to this report .
Ukrainian ##s march in Kiev protesting deal over Russian military presence . Deal over Russia 's lease of naval base in Black Sea port for 25 years . In exchange , Ukraine gets 30 percent increase in price of natural gas from Russia . Ukraine and Russia had been at odds during previous Ukraine administration .