Manchester United swooped for Edinson Cavani among four signings on Monday, but Arsenal completed the biggest deal of transfer deadline day with the arrival of midfielder Thomas Partey from Atletico Madrid.
United were under the spotlight to respond to a humiliating 6-1 home defeat to Tottenham in the Premier League on Sunday.
Dutch international midfielder Donny van de Beek had been the only addition to Ole Gunnar Solskjaer's squad prior to Monday's arrival of Cavani, Alex Telles, Amad Diallo and Facundo Pellistri.
Cavani, 33, has been a free agent since leaving Paris Saint-Germain in June and signed a one-year deal with the option for a second season.
The Uruguayan scored 200 goals in seven seasons in the French capital to become the club's all-time top goalscorer.
"Manchester United is one of the greatest clubs in the world, so it is a real honour to be here," said Cavani. "I've worked really hard during the time off and I feel eager to compete and represent this incredible club."
The arrival of left-back Telles from Porto for £13.5 million (US$19 million, 15 million euros) is arguably even more important to aid a United defence that has conceded 11 goals in three Premier League games to start the season.
"He is a player we have been tracking for some time and his performances over the past few years are exactly what we are looking for," said Solskjaer on the Brazilian international. "He is a fighter and a winner and will add real determination and competition to the squad."
After a forlorn chase of Borussia Dortmund's Jadon Sancho, United also secured two promising 18-year-old wingers in Uruguayan Pellistri from Penarol and Atalanta's Diallo, who will join in January in a deal that could reportedly reach up to 41 million euros.
Partey has been a long-term target for the Gunners, who have matched his 50 million euro (£45 million, US$59 million) buyout clause to bolster their threadbare midfield options.
The Ghanaian has made 188 appearances for the Spanish giants and was part of the Atletico sides that reached the 2016 Champions League final and beat Arsenal on the way to winning the Europa League in 2018.
"We have been watching Thomas for a while, so we're now delighted to add such a high quality player to our squad," said Arsenal boss Mikel Arteta. "He is a dynamic midfielder with great energy."
Uruguayan international Lucas Torreira has gone the other way from Arsenal to Atletico on a season-long loan deal, while another frozen out midfielder at the Emirates, Matteo Guendouzi, joined Hertha Berlin on loan.
Premier League table-toppers Everton signed centre-back Ben Godfrey and Swedish international goalkeeper Robin Olsen on loan from Roma.
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