But the two main suburban rail lines called RER A and B, which connect central Paris with Disneyland Paris and the Charles de Gaulle and Orly airports, saw more severe disruptions.
Trains on the metro lines still open were packed, with some running only every 15-20 minutes instead of the usual three-minute rhythm.
"It's a mess," said Sylvie, 46, after failing to board a metro on the number seven line because of the crowds.
Authorities in London also said the Underground system was "severely disrupted", with limited or no services running, and advised people to avoid trying to use the network.
Reports said many buses were packed to capacity, while roads were more congested than usual.
French unions have staged strikes across several sectors in recent weeks seeking pay hikes or increased hiring as spiralling energy costs feed into widespread inflation.
Thursday's strike included a protest march in the capital that shut down major traffic avenues.
But the Paris transport strike did not spill over into other sectors, with only the hard-line CGT union calling for general work stoppages.
"Let's unite, let's be as many as possible, even if we have to return to the roundabouts," said Communist Party leader Fabien Roussel, a reference to the "Yellow Vests" movement starting in 2018 that had anti-government protesters gather at busy traffic circles.
Unions representing the RATP's nearly 70,000 employees say they are feeling the pinch of soaring prices, but are also overstretched because of insufficient hiring, resulting in increased sick leave.
That has led to more service delays or lower frequency on busy metro lines in recent months, causing headaches for the system's roughly 12 million daily users.
In neighbouring Spain, meanwhile, an association of independent truck drivers called for an unlimited strike starting Monday to protest against delays in the implementation of a charter protecting their incomes.
The call comes eight months after an earlier strike in the sector had a major impact on the Spanish economy.
