Basílica de Nossa Senhora dos Mártires (Basilica of Our Lady of the Martyrs) is a Neoclassical…read morestyle church, originally built in 1147, destroyed, like so much else, in the 1755 earthquake, and rebuilt. A very similar story can be told about the church a block up the street, Igreja Nossa Senhora da Encarnação. Both churches were rebuilt in the style of the post-earthquake era, but Nossa Senhora dos Mártires has far more back history, having existed for centuries before Senhora da Encarnação.
This church is dedicated to the martyrs who participated in the 1147 reconquest of Lisbon from the Moors. It began as a small chapel built to house the image of the Virgin brought by English crusaders, and it was where the first baptism took place after the expulsion of the Moors. It is better known to the Portuguese as the site where poet, Fernando Pessoa, was baptized.
This church has free admission, but you must observe open hours. If you participate in a service, you will hopefully get to hear the church's impressive 18thC organ, one of the best in Portugal.
As mentioned, about a block away on the same street is Igreja Nossa Senhora da Encarnação, so it is convenient to visit both churches at once. Afterward, I recommend a nata (egg tart) at Manteigaria, on the far corner of Praça Luís de Camões.