The Europeana website was attracting more than 10 million hits an hour – more than double the number which had been anticipated.
The site includes paintings, photos, films, books, maps and manuscripts from 1,000 museums, national libraries and archives across Europe.
It is expected to reopen in December after technological improvements.
Users clicking on Europeana.eu currently find a message saying the site is “temporarily not accessible due to overwhelming interest after its launch”.
It adds: “We’re doing our utmost to reopen Europeana in a more robust version as soon as possible. We’ll be back by mid-December.”
“Thousands of users were searching for the words ‘Mona Lisa’ at the same time”, explained a spokesman for the European Commission.
“It confirms it’s worth doing, European culture is more popular than we had anticipated in our wildest dreams,” he said.
After a massive surge just before Europeana’s launch, the system’s creators doubled the number of servers from three to six and got it working again for a short time.
However they will now perform more tests to ensure the digital library can stay open at peak times.
On Thursday, most hits came from Germany, followed by France and Spain.
However, 4% of online requests about Europe’s cultural heritage came from the United States.