In a campaign tagged "We deserve better", about 1,000 people came out in the western city to chant "neither Le Pen nor Macron".
Riot police used tear gas to stop them reaching the historic city centre.
The far right's Marine Le Pen has spoken in Nice and centrist Emmanuel Macron visited a troubled Paris suburb.
Opinion polls taken since the first round on Sunday suggest Mr Macron, candidate of the En Marche (On The Move) movement, will easily beat Ms Le Pen, who has temporarily stood down as leader of the National Front (FN), in the second round on 7 May.
However, Ms Le Pen upstaged her rival on Tuesday when she turned up in his northern home town of Amiens just as he was visiting himself.
Ms Le Pen found herself under fire again on Thursday as EU sources accused her of defrauding the European Parliament of about €5m (£4m; $5.4m), allegedly paying FN assistants who were not really working for MEPs but were engaged in party work in France.
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Last updated  April 25, 2017
The polling average line  looks at the five most recent national polls and takes the median value, ie, the value between the two figures that are higher and two figures that are lower.
In Rennes, between 950 and 1,500 demonstrators marched in the city centre, French media report, after a peaceful rally.
When a group of about 50 tried to occupy the railway station, police used tear gas and one officer cut off from the rest briefly drew his pistol to protect himself, Le Parisien daily reports (in French).
Unrest continued in the city centre, with police bringing in a helicopter to survey the crowd, AFP news agency says.
Placards read "Expel Marine Le Pen, not immigrants" and "We don't want Macron or Le Pen".
Other unrest was reported in the western city of Nantes and parts of the capital Paris.
Since Mr Macron and Ms le Pen narrowly won the first round on Sunday, some supporters of the losing candidates have advocated a protest vote against both on 7 May.
Mr Macron visited the deprived Paris suburb of Sarcelles on Thursday to meet local people in a stadium where he played football briefly with delighted children.
Sarcelles, home to large Muslim and Jewish communities, saw riots in the summer of 2014 in which Jewish-owned businesses and a synagogue were targeted.
Mr Macron accused Ms Le Pen of not being willing to visit "a district like this". "France is not hatred and rejection of others," he said.
Speaking later in an interview for the TF1 channel, he said France's biggest challenge was mass employment and it needed a policy "which allows companies to hire and invest".
He promised, if elected, fundamental reform of labour laws this summer.
Elsewhere in the interview, Mr Macron vowed to:
Ms Le Pen gave her first big rally of the second round in the southern city of Nice, which was traumatised by the Bastille Day lorry attack claimed by so-called Islamic State last year.
She accused Mr Macron, a former banker, of being the candidate of the "oligarch class" whose idea of France was a space where "everything can be bought and sold".
Portraying herself as a patriot to the cheering crowd, she said the election was a "referendum for or against France" and she promised to halt "mass immigration".
"I will give France back its borders immediately because I choose France," she said.
Describing Nice as a city "martyred by Islamist terrorism", she said she would show no "weakness in the face of Islamic fundamentalism".
The rally ended with a rendition of the national anthem, La Marseillaise, which was sung twice at Ms Le Pen's request from the stage. "Forward to victory, my friends!" she cried.
Local FN representative Lionel Tivoli told Reuters news agency earlier that party membership in Nice's Alpes-Maritimes region had jumped from 740 two years ago to 3,500-4,000 now, driven in particular by the attack.
However, sports coach Dominique Eche, 62, whose children narrowly escaped the attack, told Reuters:   "I saw the Nice attack from the inside and I find it appalling to try and benefit from such attacks, to say: 'It wouldn't have happened if I'd been in power'."
Teenagers in Rennes and other French cities have held rallies or blocked schools in a protest against both presidential candidates.