The two Algerian nationals escaped from a cargo ship that was carrying gas and which had docked in Barreiro, on the south side of the Tagus River.
According to newspaper Correio da Manhã, the pair swam across the Tagus to the Portuguese capital.
The newspaper reports the two Algerians were stowaways on the ‘Queen Isabella’ tanker after sneaking onto it in Algeria.
After docking in Portugal, they reportedly broke a window and leapt from the vessel into the waters of the Tagus, hence making their bid for freedom.
The newspaper further said Portuguese authorities had photos of the clandestine immigrants and had issued a general alert.
At the time of going to press, authorities were still trying to locate them.
In recent months several Algerian nationals have illegally entered Portugal after managing to dodge security at Lisbon airport while in transit.
In January, the Christian Democratic Party (CDS) described the number of foreign citizens, mostly Algerians, absconding from Lisbon Airport as “abnormal” and said it is an issue of national security that requires an “urgent response” from the government.
The centre-right party’s comments were made after two Algerian men absconded from Lisbon airport on 12 January.
In October 2016, the Immigration and Borders Service SEF and the PSP police prevented a number of Algerian nationals who had just landed in Lisbon from Morocco, from escaping.
A group of six Algerians were detained at the airport after trying to escape from a moving aircraft, and were to be expelled from Portugal without facing trial. The six were arrested after trying to open the emergency exit of an aircraft that was in movement on the Humberto Delgado airport’s tarmac.
The group had earlier arrived in the Portuguese capital on a flight from Casablanca (Morocco) to Algiers (Algeria) via Lisbon.
They were detained by PSP officers inside the aircraft, after attempting to open an emergency exit to flee the plane while in Europe.
The month before, an Algerian citizen, travelling between Algeria and Casablanca, illegally absconded from Lisbon airport.
And at the end of July 2016, four men were detained by the PSP at Lisbon airport for violating security rules when they tried to avoid passport control by running across the airport runway.