Protests and strikes against unpopular pension reforms kicked off again Tuesday across France, with police security ramped up against feared violence and government warnings that radical demonstrators intend "to destroy, to injure and to kill."

Fears that violence could mar the demonstrations prompted what Interior Minister Gérald Darmanin described as an unprecedented deployment of 13,000 officers, nearly half of them concentrated in the French capital.

Protests got underway peacefully Tuesday morning, with large crowds in multiple cities. In Paris, striking railway workers with burning flares and flags invaded and blocked train tracks serving one of the capital's main stations, Gare de Lyon.

Police were braced for violence later in the day. The interior minister said more than 1,000 "radical" troublemakers, some from overseas, could latch on to marches planned in Paris and elsewhere.

"They come to destroy, to injure and to kill police officers and gendarmes. Their goals have nothing to do with the pension reform. Their goals are to destabilize our republican institutions and bring blood and fire down on France," the minister said Monday in detailing the policing measures.

Union leaders and political foes of President Emmanuel Macron blame his government for protest violence that has flared in recent weeks, saying his push to raise France's legal retirement age from 62 to 64 sparked it.