Municipal Company EGEAC, which manages the visits, said due to demand new enrolments could be opened.
Access to the online enrolment form had blocked due to the sheer number of people looking to sign up for a visit, and the spaces available filled up within hours of the technical glitch being fixed.
According to EGEAC, people who had tried to enrol online when spaces on the guided tours were opened on Monday afternoon were contacted via the website, telephone or Facebook and asked to try again.
The technical glitch was resolved some 24 hours later and spaces filled within hours, but, “given the volume and interest shown, the company will extend visiting times and address the possibility of new reservations [next] Monday.
“While we may try to increase the number of visits, access to these galleries imposes some limitations relating to security and climate conditions, and therefore it will always be difficult to satisfy demand”, the company explained.
Located beneath Rua da Prata in Lisbon’s city centre, the hidden Roman Galleries are one of Portugal’s most remarkable heritage sites and open only for a few days a year, usually in September. The Galleries were unearthed during the rebuilding of Lisbon following the devastating Great Earthquake of 1771.
Known for being one of Lisbon’s best secret attractions, the tunnels are normally flooded, but once a year the fire brigade drains the compartments to allow thousands of visitors to access the unique underground tourist attraction on free guided tours.