Tremors struck the popular tourist town at 7.44am local time, according to Portugal‘s Institute of the Sea and Atmosphere (IMPA).
The quake, which measured 4.3 on the Richter scale, was felt in nearby towns and cities including Almada and the Portuguese capital, Lisbon.
Joao Tiago tweeted: “My cat seems to have felt it in Almada! She woke me up completely crazy!”
Diogo Rodrigues, who lives in Alges, wrote: “I heard in the rumble while I was in bed and that’s what got me up.”
Another Twitter user added: “I live in Parque das Nacoes and felt the earthquake. I was still lying down and I felt the bed tremble.”
And Luis Simoes noted: “I woke up a few seconds earlier, I heard a thunder-type boom and everything shook right away.”
A spokesman for IMPA said: “This earthquake, according to the information available to date, caused no personal or material damages.
“It was felt with maximum intensity IV (modified Mercalli scale) in the region of Sobral de Monte Agraço. It has been felt even less intensely in Lisbon.”
The Mercalli intensity scale has 12 levels, with a level three corresponding to a weak earthquake.
Around 2.5 million British tourists visit Portugal every year, according to the Foreign Office.