Activists and trade unions held rallies across Europe and the rest of the world to mark May Day, the International Workers Day.
Most events were expected to be peaceful protests for issues like workers' rights and world peace.
A rally in Turkey, however, turned violent as thousands of police and demonstrators clashed in Istanbul after crowds tried to defy a government ban to march to the city's iconic Taksim Square.
Security forces pushed back demonstrators using water cannons and tear gas. Protesters retaliated by throwing stones and hurling firecrackers at police.
Police made several arrests and protesters blocked some roads near the square with burning trash bins. Some were seen firing firecrackers and throwing stones at police.
The government had deployed some 10,000 police officers in and around the square, where police clashed with protesters last year.
In Iran, thousands of workers held a May Day demonstration in Tehran to demand improved conditions and protest against foreigners taking jobs.
Iran's semi-official ILNA news agency reports demonstrators gathered in the center of the capital near the offices of the House of Labor, the official workers' union. Slogans on some banners read: "Employing foreign workers amounts to putting Iranians out of work."
Iran suffers from unemployment of more than 10 percent of the workforce. The country has more than a million workers from Afghanistan, employed mainly in construction and agriculture.
In Moscow, tens of thousands of workers braved chilly rain to march across Red Square. Instead of the red flags with the Communist hammer and sickle used in Soviet times, they waved the blue flags of the dominant Kremlin party and the Russian tricolor.
Despite an economic crisis that is squeezing the working class, reports said there was little if any criticism of President Vladimir Putin or his government.
Participants said the May 1 march was a tradition going back to their childhood. This Soviet nostalgia was imbued with feelings of patriotism in the run-up to the 70th anniversary of victory in World War II on May 9.
The Communist Party later held a separate march under the slogan "against fascism and in support of Donbas," with participants calling for greater support for the separatists fighting the Ukrainian army in the Donbas region of eastern Ukraine.
With reporting by Reuters, dpa, AP and AFP