About 10 feet tall, the catapult was piled with bags full of cobblestones to send whistling into the ranks of the police.
Men in masks hovered around it but did not let loose, apparently fearful of hitting protesters, given the large crowd nearby.
Clashes between demonstrators and the police continued Monday after fighting broke out the day before between riot police officers and protesters who had gathered to object to new laws limiting public assembly.
A struggle began for a small swath of a side street near Independence Square, the center of the protest.
As the police have escalated their use of force — hurling ostensibly safe stun grenades and firing rubber bullets that have maimed at least four people and injured scores of others — so have the protesters.
“We’re on a crusade now,” one man wearing a balaclava said, pointing proudly at the scrap-wood catapult, designed to fire cobblestones about 200 yards down range into the police, with presumably devastating effect.
The protests began in November after President Viktor F. Yanukovich declined to sign a free-trade agreement with the European Union, negotiating a financial aid deal with Russia instead.
A movement that seemed to be fading was re-energized by opposition to new laws against public assembly passed last week.
“We stood, we asked peacefully, but the government didn’t hear us,” said Svyatislav Y. Shamis, 32, a lawyer, while watching preparations to fire the catapult.
“The Parliament voted for these unpopular laws, blatantly violating human rights.
They constantly steal, and they pass laws for themselves and their businesses.”
In a sign of the darkening mood, two dazed men walked into a group of demonstrators on Monday morning naked, barefoot and covered in welts, a video of the scene showed.
The men said they had been detained by riot police officers, stripped of their clothes and shot multiple times at close range with rubber bullets, then let go as a warning to others.
The newspaper Ukrainskaya Pravda said at least 11 protesters had been abducted by unknown men on side streets near the square since Sunday.
The Interior Ministry published an order Monday that authorized the police to use lethal force if needed, though there was no indication that they had done so on Monday.
Protesters threw firebombs made from beer bottles and gasoline.
In the cold evening air, they arced in curlicues of sparks and then burst into fire on the shields of the riot police.
Opposition leaders including Arseniy P. Yatsenyuk, a leader of the Fatherland Party, and Vitali Klitschko, a former heavyweight boxing champion and the leader of the political party Ukrainian Democratic Alliance for Reform, have renounced the violence but have been powerless to stop it.
The opposition has long expressed fears that the government would use an incidence of violence as a pretext to clear the otherwise largely peaceful protest movement from Independence Square.
A planned meeting on Monday between these leaders and an aide to Mr. Yanukovich intended to tamp down tensions did not take place.
Instead, Mr. Yanukovich, in a statement posted on his website, called for calm but also warned of an imminent response to the civil uprising in the capital.
“When peaceful actions have escalated into mass riots accompanied by demolition, arson and violence, I am confident that such phenomena threaten not only Kiev but the whole of Ukraine,” he said.
“I am convinced that you are the ones who clearly understand that we must try to save our country together, to increase its power, authority and opportunities.”
On the street, his message seemed to go unheeded.
Hanna Hrabarska, an independent photographer, described the scene on her Facebook page:
“There are all sorts of people there now: radicals, nationalists, teenagers with baseball bats, babushkas beating barrels with sticks, Cossacks picking up the cobblestones that are not dismantled yet. So, if you have weak nerves, better not to go there.”
Source: The New York Times