Crowds of environmentalists, anti-establishment activists and angry youths gathered in demonstrations across the globe on Friday -- May Day, also known as International Workers' Day. In many cases, demonstrations turned violent and police used tear gas and other measures to break up unruly crowds.

In many former communist states, May Day remains a national holiday that’s peacefully celebrated, but in other countries like Italy and Turkey, demonstrators clashed violently with police. The streets of Milan and Istanbul were shrouded in smoke and tear gas as police tried to fend off angry protesters, many of them outfitted with gas masks and fireworks.

Istanbul’s clashes came a month after the Turkish parliament passed a law that allows police to search and detain protesters at will. Police arrested around 140 people on Friday near Taksim Square. The government shut down access to the square, which is a popular protest site. Mass demonstrations against President Recep Erdogan were held there in 2013 and also ended violently. Protesters set off fireworks and threw stones at police, who responded with tear gas, water cannons and flashbang grenades. Eighteen demonstrators and six policemen were hurt in the clashes. A protest in Ankara attended by tens of thousands of demonstrators went on without serious incident. RTX1B3H3 A protesters attempt to pull a police bus which blocked the road connecting to Presidential Blue House during a May Day rally in central Seoul, South Korea, May 1, 2015. Tens of thousands of workers protested against the government's push to cut pensions and ease rules for firing employees, according to local media. Photo: Reuters/Kim Hong-Ji

Milan’s protesters were out in force to oppose the 2015 Milan Expo, an expensive world's fair that many in Italy say is a waste of their debt-ridden country’s money. More than 140 countries have exhibitions at the six-month event, which features the latest in tech and business initiatives from each country, often displayed in glamorous pavilions. But just outside the expo’s peaceful grounds, protesters armed with self-made batons and sporting motorcycle helmets (some also wearing gas masks) faced off against riot police armed with water cannons and tear gas. Protesters set cars on fire and tossed Molotov cocktails at police.

An event in Donetsk, Ukraine, sanctioned by the government of the self-declared Donetsk People’s Republic became violent when protesters sporting Soviet-era flags and paraphernalia marched on the central police station in the city, which up until Friday was manned by policemen with no particular allegiance to either the DPR or the Ukrainian government. The crowd demanded militiamen with the DPR be allowed to patrol as police. Militiamen entered the station and “negotiated” successfully to join police patrols, and protesters mounted DPR flags on the building. The protesters then moved to the general prosecutor's office, where they tossed pieces of pavement at the building and were met with tear gas and flashbang grenades thrown by law enforcement officials. After a number of people were wounded, the crowd dispersed.

Clashes broke out in South Korea and Greece as well. Tens of thousands of Korean demonstrators, mostly from a national trade union federation, were out to show their opposition to the government’s handling of last year’s Sewol ferry disaster that killed 300 people. They tried to overturn a bus. but were pushed back by police. No injuries were reported. In Greece, anti-austerity protesters threw gas bombs at police but no one was arrested.