At least 20 people were injured as more than 60,000 unionized workers marched on Tuesday in protest against proposed Belgian labour-market reforms including longer hours and a higher retirement age, snarling up traffic in a large part of Brussels.

60,000 union workers march against labour reform plans in Brussels as protests turn violenthttps://t.co/SBwOd6w2EI pic.twitter.com/6CmBbHikwu

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Police said 20 people were injured in clashes when protesters threw rocks and firecrackers at riot police, who responded by firing water cannon. Police arrested 23 people, a spokeswoman said.

The protests are the first in a series of actions planned for the coming months, including a strike on June 24 and a day of national demonstrations on Oct. 7, marking two years since the centre-right government took power. "It's always in the same purses that they come to find money," said Marie-Claire Legros, a protester from Grace-Hollogne in the province of Liege.

Trade union workers across a wide range of sectors are upset over proposed reforms including a planned wage hike that was scrapped, higher VAT on electricity, a retirement age of 67 and extending the maximum workweek to 45 hours from 38.

Tuesday's march cut across the capital and caused a temporary shutdown of certain transport tunnels and train stations.