LONDON (Reuters) - Workers, students and leftists took to the streets for May Day rallies that stuck to tradition but also showed how the world is changing.

In communist Cuba, whose vast holiday parade has been a fixture for years, illness kept veteran leader Fidel Castro away for the first time in decades.

"We need him to return," said Luisa Cuellar, who rose before dawn to walk with friends to Havana's Revolution Square, the political heart of Cuba. "He is the one who keeps us united."

Police fired in the air to disperse crowds demanding labor rights in the Macau enclave of communist China, while in the United States, where May Day is usually ignored, activists planned to march for illegal immigrants' rights.

Protesters called for better pensions in Greece, work safety in Italy, more HIV medication in Zimbabwe and anti-American defiance in Cuba, where all citizens are expected to march.

Apart from Macau, clashes also flared in Istanbul, Tehran and Berlin but there were few of the street battles seen in Europe in recent years between police and opponents of globalization.

Police beat and detained hundreds of leftist demonstrators in street fights in Istanbul, in a Turkey already tense after army threats and a million-strong march by secularists opposed to what they see as an Islamist presidential candidate.

Riot police fired tear gas and used water cannons to break up the crowds. Youths threw Molotov cocktails and burnt cars.

In Tehran, protesters seeking better pay and the resignation of the labor minister clashed with police after a rally at a sports stadium, where thousands had earlier protested against government economic policies. There were no reports of injuries.

"They're not paying for our hard work," said protester Ahmad Mirbakhsh, a May Day complaint shared by many around the world.

LONG LIVE THE WORKERS

Venezuelan President Hugo Chavez chose the day of the worker to strip the world's biggest multinational oil companies of operational control over massive Orinoco Belt crude oil projects, a vital move in his nationalization drive.

"The wheel has turned full circle," he told a rally, hailing the takeovers as the South American nation reclaiming its sovereignty. "Long live the workers."

In Germany, union leaders said half a million people joined rallies and criticized Chancellor Angela Merkel's conservative party for resisting efforts by the Social Democrats' coalition partners to introduce a minimum wage.

"Everyone has to profit from the country's economic upturn -- and not just the rich and high-wage earners," said union leader Michael Sommer.

In Berlin, which has faced May Day violence every year since 1987, police detained 119 people for acts such as throwing bottles, stones and firecrackers at authorities. A spokeswoman said 15 police officers were injured.

At Moscow's biggest march, 20,000 people demanded higher wages and pensions and better working conditions.

About 5,000 Communist supporters waving red Soviet flags and holding pictures of Vladimir Lenin and Josef Stalin gathered outside a statue of Karl Marx in central Moscow. They called for mass protests to return socialist rule in Russia.

In Italy, half a million people gathered in Rome for an annual holiday rock concert and a further 100,000 waved red flags at a rally in the northern city of Turin.

One place without the usual street rallies this year was Bangladesh, where they are banned under a state of emergency.

Instead, the army-backed interim government urged labor groups to free themselves from the yoke of political parties.