The marches come a week after hundreds of thousands of people -- 200,000 according to the police, half a million according to organisers -- demonstrated against the plan, in the first challenge to Macron since he was elected in May.
More rallies are expected on Saturday, staged by hard-left political party France Unbowed, in another measure of the resistance to 39-year-old Macron's pro-business agenda.
Philippe Braud, a professor emeritus of Sciences Po university, believes the government has the upper hand and that the protest movement in France "has been weakening for the past 10 years".
Today, "there's a sort of resignation among the French to reforms that are seen as necessary", Braud told the AFP news agency, adding that their passage will be a "big victory for Macron".
Thursday's protests have been called by France's biggest trade union, the CGT, and the smaller Solidaires union, which object to new powers being given to employers and the changes being rushed through parliament.
While some student and teachers groups are set to join them, the leaders of rival powerful trade unions such as the CFDT and the FO have declined to join the protests for a second week.
The changes, which are being fast-tracked via executive orders, are designed to give employers more flexibility to negotiate pay and conditions with their workers while reducing the costs of firing staff.
Labour Minister Muriel Penicaud, who will present the reforms to the cabinet on Friday, said last week: "We will not back down."
Public opinion is divided, according to a recent BVA poll, with most respondents saying they think the reforms will boost France's competitiveness but fail to improve employees' working conditions.
The Macron team insists that the reforms will encourage hiring and will offer the best cure to France's stubbornly high unemployment rate, which stands at 9.5 percent, roughly twice the levels in Britain or Germany.
Once cabinet approves the measures, they are expected to be published in the official gazette and enter law.
The use of executive orders is a way to pass the measures quickly and avoid a prolonged battle in the streets -- as seen last year when Macron's predecessor François Hollande made similar changes.
While the strategy may succeed in overhauling France's rigid and complex labour laws, some critics see the method as reinforcing perceptions of Macron as a monarchical or even "pharaonic" leader.
Criticism of him as aloof and sometimes authoritarian, as well as a series of missteps, have erased Macron's honeymoon period, with his approval rating languishing at 44 percent according to an Odoxa poll out on Monday.
But Braud believes Macron "doesn't care about his popularity, knowing that he won't be defeated in the street".
Jean-Luc Mélenchon, the head of France Unbowed, has slammed the reforms as no less than a "social coup d'etat". Popular among students and workers, he has an approval rating of 68 percent in the Odoxa poll.
The labour reforms, which have broad support from other EU countries, are crucial to Macron's wider plans for the 28-member bloc, for which he advocates major institutional changes.