The debate opened on the topic of universal basic income, the central campaign theme of former education minister Benoît Hamon.
This idea, that the government should unconditionally and regularly pay a sum to all citizens, regardless of income, is strongly opposed by Hamon’s rival, former prime minister Manuel Valls.
“I think Benoît’s message here is terrible," said Valls. "He thinks we should give up on jobs, that we should accept unemployment, and that everyone should get the same sum regardless of income."
"Finally, the cost is exorbitant,” he concluded.
Hamon argued that a universal basic income would offset the effects of the increasing automation of work due to technology.
Different views on secularism
Universal basic income wasn’t the only subject the candidates sparred over.
In the days before the debate, Valls also accused Hamon for being "ambiguous on radical Islam" and secularism.
Hamon responded by saying he defends secularism, but also the right of individuals to freely express their beliefs.
“Secularism shouldn't be dogmatic," said Hamon. "It shouldn't be the religion for those who have no religion."
"When a Muslim woman freely decides to wear a veil, which is completely possible, it doesn’t matter what we think. She has the right to do so under the law."
"I want to protect this liberty,” he added.
'Do you speak English?'
At one point during the debate the moderators asked questions that came from the audience.
The first one was whether the candidates spoke English.
Hamon replied that he spoke fluent English, only to immediately translate what he had just said in French.
Valls, who was born in Barcelona, had a different response.
"My English is very bad, but I speak well Spanish," he said.
The second and final round of the primary vote this Sunday will determine whether Hamon or Valls will be the Socialist candidate in this spring’s presidential election.