The ramped-up border security measures, first touted last week, will see officers carrying out random searches in the lead-up to the crucial COP 21 talks on climate change in Paris.
“We are going to step up these controls ten days before the demonstrations on November 29th and December 12th, and four days before the demonstrations those controls will be more systematic,” French interior minister Bernard Cazeneuve said on Thursday.
Cazeneuve insisted France - contrary to reports - was not closing its borders, saying the month-long controls were in line with Schengen rules and would not impede free movement across France's borders.
But the move to strengthen border security comes three weeks too late, with violent “black bloc” protesters already in the country, police sources have told France's RMC radio station.
The police source told the radio station the black bloc protesters - whose name derives from the black ski masks, scarves, sunglasses and motorcycle helmets they wear to disguise their identity and protect them from tear gas attacks - had already arrived in France.
The protesters had come from countries as diverse as Germany, the UK, the Netherlands, Belgium and even the Philippines, the source said, adding that security agents already had them under surveillance.
The aim of COP 21 is to unite all the world's nations in a single agreement on tackling climate change, with the goal of capping warming at two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels.
Lead-up talks were held in Paris this week with French foreign minister Laurent Fabius saying progress had been made on several key points.
But France and the United States appeared to clash on Thursday over the question of whether any eventual COP 21 agreement would be legally binding, with French President Francois Hollande saying "If there is not a binding accord, there will not be an accord".
A day earlier, US Secretary of State John Kerry said the United States would not sign a deal in which countries were legally obliged to cut greenhouse-gas emissions.
In a joint statement signed in November, Hollande and his Chinese counterpart Xi Jinping had called for "a Paris accord that is ambitious and legally binding".