Paris: French trade unions clashed here during the annual May Day marches ahead of the upcoming second round of the French presidential elections, a minister said.

Several of the union marches ended in clashes on Monday clearly bringing out their differences before the poll scheduled for May 7, Efe news reported.

According to the French Interior Minister, Matthias Fekl, six riot police were injured. Some of the trade unions opposed the right-wing presidential candidate, Marine Le Pen, Fekl said.

The police arrested five persons following the incident.

It all started when dozens of people - either covering their faces or wearing helmets - threw Molotov cocktails and stones at the riot police, who then responded with tear gas grenades.

Fekl said that the incidents were produced by "several hundred professional vandals who came with only one goal: to attack security forces, to smash a policeman and wreak damage."

Some of them carried placards denouncing the two options presented -- the Socio-liberal Emmanuel Macron and the Right-wing extremist Marine Le Pen.

This, however, differed from the organisers' view. Although they were against Le Pen but did not claim an explicit support for Macron and his socio-economic policies, or his intention to reform the labour market.

According to the Interior Ministry, around 142,000 people participated in the nationwide demonstrations. Although a major trade union claimed that figure was 280,000.