Left-wingers protested in Paris on Thursday one day after a group of far-right militants beat a leftist student to death in a busy central Paris district.
Clement Meric, 19, a student at the prestigious Sciences Po university, was beaten up near St. Lazare station and taken to hospital with head injuries. He was declared dead on Thursday.
Several hundred people gathered near the site of the beating where they chanted left-wing slogans and friends made speeches paying tribute to Meric.
Another demonstration took place in the heart of the Latin Quarter, the capital's traditional student neighbourhood. Separate protests were held at Meric's university and in some other French cities.
Three men and a woman aged between 20 to 30 were arrested on Thursday afternoon in a Paris suburb, the Paris prosecutor's office said, and three other arrests were made later on.
Police said the perpetrators appeared to belong to a fringe far-right group called the Revolutionary Nationalist Youths, although that group's head, Serge Ayoub, denied it was involved.
"An extreme right group is behind this murder," Interior Minister Manuel Valls told reporters.
France has seen violent protests against the legalisation of gay marriage in recent months, as far-right groups armed with bricks, batons and firecrackers hijacked a series of street marches.
President Francois Hollande, on an official visit to Tokyo, condemned Wednesday's attack and Prime Minister Jean-Marc Ayrault told parliament the government would step up plans to clamp down on far right groups.
National Front leader Marine Le Pen condemned the killing and said her party had nothing to do with it.