Around 230 of the gallery's 600 employees are taking part in the strike, mainly members of the PCS union.
A strike that has kept parts of the National Gallery in London shut for months passed the 100-day mark today, causing confusion among some visitors to one of Britain's premier cultural attractions.
The museum on Trafalgar Square has remained open during the strike but around half the building was shut today including galleries with works by Caravaggio, Rembrandt and Titian, an AFP reporter saw.
Strikers are protesting against the introduction of private companies to provide services such as security and ticket sales at the museum, which in a normal year receives around six million visitors.
At a picket outside the museum's grand colonnade entrance, staff handed out leaflets in English, French, Italian and Spanish, asking tourists to express their solidarity by boycotting the museum and signing an online petition to the management.
"We are asking you to show your support by not visiting the gallery today and go to another London museum instead," said the leaflet, published by the Public and Commercial Services Union (PCS).
A sign by the entrance read: "Due to industrial action there are some room closures today".
Ieva, a 25-year-old from Lithuania visiting the museum for the first time, expressed her frustration.
"The closures bothered me because I couldn't see the Rembrandts and some other famous paintings," she told AFP.
But others said they were not particularly affected.
"It didn't really bother us," said Sara and Marco, an Italian couple who seemed happy to spend the extra time outdoors on a rare sunny day in London.
Jakub Vavra, a 20-year-old Czech tourist, added: "It didn't bother me because I came to see the Impressionists and I could."
'A Precious National Asset'Protests by National Gallery staff began last year and intermittent strikes have continued since then, although the latest one has been by far the longest.
During the latest 100-day stint, the amount of the gallery closed by strike action has varied day-to-day with private contractors being brought in to plug gaps.
Jeremy Corbyn, the new leader of Britain's main opposition Labour Party, endorsed the strikers during a speech shortly after his remarkable landslide election this month.
"They are doing their best to defend something we all own, know and love," he told the Trades Union Congress.
He hailed the workers as "a precious national asset, not something to be traded away on the market of privatisation."
Around 230 of the gallery's 600 employees are taking part in the strike, mainly members of the PCS union.
"We are optimistic. We are making progress and we are doing proper negotiations," said Clara Paillard, head of the union's culture section.
The National Gallery's 2,300-painting collection is recognised as world-leading, spanning from the 13th to the 20th centuries including masters such as Botticelli, Rubens and Vermeer.
Management has complained about the length of the strike and says it has offered fair conditions to its staff.
"This is the 101st day of strike action over this issue," the museum said in a statement, adding that privatising some services "would enable us to operate more flexibly".
As a public asset, it said, the museum had to be "accessible as much as possible to as many people as possible."