Manchester United, Arsenal and Everton were among a host of clubs scouting at Benfica's 2-1 derby win over Sporting Lisbon on Sunday, sources close to the Eagles have told ESPN FC.
Scouts from United have been regular visitors to Portugal this season as they keep a close eye on Benfica trio Victor Lindelof, Nelson Semedo and Goncalo Guedes.
Red Devils boss Jose Mourinho is actively looking to bolster his defensive options in January and Swedish centre-back Lindelof is reportedly among his top targets.
Right-back Semedo is also on United's radar as Mourinho looks to bring in a new full-back with Antonio Valencia filling in this season.
Sunday's game at the Estadio da Luz also allowed United scouts to cast an eye over Sporting's Gelson Martins, with the talented winger a long-term target for the club after watching him on several occasions this term.
Sources said the reason for Arsenal's visit was to check on Guedes, who is one of the most sought-after young players in Europe after impressing for Benfica over the last 18 months.
Guedes is also being watched by United and the 20-year-old attacker is regarded as one of the best prospects to emerge from Portugal in recent years.
Premier League clubs Everton and Southampton, as well as Championship duo Sheffield Wednesday and Norwich, were also represented at the game as they looked at potential targets.
French champions Paris Saint-Germain, Italian giants Juventus and AC Milan and La Liga outfit Atletico Madrid were also in attendance at the game as most of Europe's big clubs keep tabs on Benfica and Sporting's best talents.
Benfica will be reluctant to lose any of their top stars in January as they prepare to take on Borussia Dortmund in the round 16 of the Champions League in February and March and they are under no pressure to cash in on any of their players.
Sporting, however, may face a battle to keep such players as Martins and Portugal international William Carvalho after they crashed out of Europe altogether last week following their shock defeat to Legia Warsaw.