Gaia firemen said they had registered a total of 21 occurrences during the early hours of Friday due to bad weather, the Douro is flooding the Areinho and Afurada areas.
"We are waiting to see what happens with the rising tide, around 09:30", the source said.
The Maritime Police told Lusa that there has been "an excessive rise in the water level" of the River Douro that is flooding "pedestrian areas" of Porto and Gaia.
The source explained to Lusa that the rise of the water was gradual, and it burst its banks around 05:30.
The Portuguese Institute of the Sea and the Atmosphere (IPMA) has issued an warning orange (the second most serious) for twelve districts of mainland Portugal and the northern coast of Madeira due mainly to rough seas.
Leiria, Santarém and Portalegre are also under orange warning due to strong rainfall forecast between 12:00 and 15:00 in the first two cases and between 12:00 and 18:00 in the case of Portalegre.
Bad weather throughout the country, caused by storm Elsa, has already caused two deaths since Wednesday, one missing and more than 50 homeless, as well as 5,400 occurrences, most of them falling trees.
Meanwhile, IPMA has also warned of the effects of another depression, called Fabien, which will hit Portugal on Saturday.
According to IPMA, the north and centre will be the areas of the country most affected by this depression, with intense periods of rain and strong gusts of wind.
However, the effects of the Fabien Depression are not expected to be the same in mainland Portugal as those of storm Elsa, "particularly in terms of wind and with more significance in terms of precipitation.
IPMA predicts a gradual improvement in the weather from Sunday onwards.