Around eight million pensioners on more than 1,200 euros ($1,400) a month will pay higher social security contributions next year as Macron seeks to slash the budget deficit.
The nine unions behind Thursday's protests say the increase — expected to bring in 20 billion euros in revenue — will set the elderly back hundreds of euros a year.
The pro-business Macron, whose government unveiled its first budget Wednesday, has made assurances that only the wealthiest pensioners will be asked to tighten their belts to ease the pressure on younger generations.
But protesters demonstrating in several cities Thursday insisted the idea they had money to spare was a myth.
“We're not all well off… and we're being hit up again,” said Danielle Bonfils, a retired secretary looking after a handicapped son in Bordeaux who struggles to make ends meet on less than 1,500 euros a month.
“At the end of the month, it's very tight,” said Bonfils, marching in the northeastern city of Reims.
- Macron's first budget: Good for the rich, bad for the old
The tax hike is aimed at funding cuts in the unemployment and health contributions of private-sector workers — one of Macron's key campaign promises.
The government has announced some seven billion euros in tax cuts on households — some of which will benefit the pensioners — and businesses in 2018 as part of a bid to spur consumer spending and entrepreneurship.
Unions and leftist parties have complained that the budget favours the wealthiest.
But Economy Minister Bruno Le Maire has insisted that it benefits all.
The pensioners are the latest group to take their grievances with France's new leader to the street.
Tens of thousands of people took part in three mass protests over his shake-up of France's complex labour code — his first major economic reform aimed at combatting unemployment of 9.5 percent.
On Thursday, the heads of France's 22 regions joined the protest movement after being told their funding from Paris would be cut by 450 million euros in 2018.
The regional chiefs slammed the decision — part of a package of 16 billion euros in spending cuts — as “unfair” and walked out of a meeting with Prime Minister Edouard Philippe.
Macron sees improving France's finances as key to winning backing in Berlin and Brussels for his ambitious EU reform plans.
The eurozone's second-biggest economy remains one of the few countries with a budget deficit above the EU-mandated three percent of GDP.
It is already set to fall to 2.9 percent this year — the first time it will have met the three-percent target in a decade — but Macron wants to trim it further to 2.6 percent in 2018.
by AFP's Clare Byrne