PARIS (AP) — Striking French train workers blocked passengers from boarding one of the few subways functioning in Paris on Monday, leading to a standoff with police on the 19th day of nationwide strikes over the government's plans to raise the retirement age to 64.

The wildcat protest by a few dozen union activists caught passengers by surprise and spilled onto the streets around the historic Gare de Lyon train station, further disrupting a key travel hub after more than two weeks of transportation trouble around the country.

Images on French media and social media showed activists jumping and chanting and waving union flags as they moved through the subway station, engulfed in red smoke from the flares they were waving.

As the protest moved outside, an Associated Press reporter saw helmeted police encircle the shouting demonstrators.

The activists targeted one of only two Paris subway lines that has continued functioning amid the strike, because it has automated trains that don't need drivers. The city transit authority subsequently shut down the line because of the protest.

The government is negotiating with unions about the retirement reform, which President Emmanuel Macron has championed to keep the state pension system financially sustainable. Unions say it threatens the idea of the welfare state.