The Egypt forward curled a low shot inside the far post in the 23rd minute to make it 29 goals in as many home matches since joining Liverpool in 2017.
United arrived on the south coast knowing a win would all-but seal second place, but a poor effort led to its first loss to Brighton since 1982.
Not even a 2-0 victory over Brighton was enough to mellow United manager Jose Mourinho, whose joy at beating Liverpool last weekend in the league was wiped out by a Champions League exit to Sevilla.
After back-to-back 3-0 losses to Manchester City across two competitions, Arsenal stumbled again following two errors by goalkeeper Petr Cech on the south coast.
Arsenal extended its unbeaten run in all competitions to seven matches to move up to fifth in the standings.
London, Aug 8 : England cricketer Monty Panesar has been fined by police for urinating in public following a nightclub brawl in the beach town of Brighton in southern England.The 31-year-old spinner has since apologised
London, Jan 5: Premier League struggler Newcastle was knocked out of the FA Cup by Brighton for the second successive season on Saturday, going down 2-0 in the third round to the second-tier club.Andrea Orlandi
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