KIEV — Ukraine’s security agency on Sunday warned of a heightened risk of terrorism, including from nearly three months of antigovernment protests. The warning raised the pressure on the opposition as Parliament tries to find a way out of the crisis.
The Security Service of Ukraine said it was putting its counterterrorism units on alert after receiving a large number of bomb threats across the country at airports, train stations, pipelines, and other locations.
In what was seen as a warning to the opposition, the agency said the seizure of government buildings also would be viewed as manifestations of terrorism.
About 30,000 people turned out for a rally on Kiev’s Independence Square, known as the Maidan, on Sunday, the day the demonstrations usually draw the largest crowds.
Opposition leaders demanded a constitutional reform that would reduce presidential powers and early elections in which they hope to unseat President Viktor Yanukovych. The measures are currently being discussed in the national Parliament, which is controlled by Yanukovych loyalists who so far have rejected those demands.
‘‘The authorities are already scared of us,’’ opposition leader Oleh Tyahnybok told the crowd. ‘‘We need to press them further.’’
In Turkey on Sunday, a state-run new agency said a court in Istanbul formally ordered the arrest of a Ukrainian man who allegedly tried to hijack a Turkey-bound flight to Sochi, Russia, as the Winter Olympics were beginning. Ukrainian officials have said the man tried to hijack the plane to press for the release of antigovernment protesters in his country.
The Turkish agency, Anadolu Agency, said Artem Kozlov was ordered jailed after questioning by police. Private NTV television said he was arrested on hijacking and hostage-taking charges.
The man claimed he had a bomb and tried to divert the Pegasus Airlines flight, which originated in Kharkiv, Ukraine, on Friday. The pilot tricked him and landed in Istanbul instead where he was subdued by security officers who sneaked on board.
The antigovernment protests in Ukraine started after Yanukovych ditched a key treaty with the European Union in favor of a bailout loan from Russia.
Protesters and police have been maintaining a shaky truce at giant barricades near a government district in Kiev for several weeks, after three activists were killed in clashes last month. Another one was found dead outside Kiev after being kidnapped from a hospital.
Many activists said Sunday they were ready to resume confrontations with police if Yanukovych refused to concede to their demands.
‘‘We are already tired of standing on the Maidan,’’ said Dmytro Shulets, 47. ‘‘Nothing happens without a fight. If talks fail, we will resort to force again.’’
During the Sunday protests in the capital, opposition leader Arseniy Yatsenyuk told the crowds that the United States and the European Union are ready to step in with financial aid for Ukraine if order is restored in the country.
Ukraine is ‘‘on the verge of default,’’ Yatsenyuk said. ‘‘The criminal regime should be removed from ruling the country as soon as possible. Our path is a new constitution, a new president, a new government, and a new nation, and we’ll win.’’
The uprising has severely affected Ukraine’s ability to raise funds. The authorities are grappling to stabilize the economy and trying to stem a currency slide.