Emergency services rushed to Streatham Vale, in south London following reports of a man in his 50s being injured.
It comes as strong winds, of up to 60mph, causing mayhem across the southern parts of the UK and thunderstorms in the north causing flooding.
A spokesman for the Metropolitan Police said: “Police were called at 1.04pm on Saturday, August 10 to Greyhound Lane in Streatham Vale following reports of a man injured by a fallen tree.
“We were called to the incident at 1.37pm. Initial reports said that a pedestrian was trapped but on arrival this was not the case”
“Officers attended with London Ambulance Service and London Fire Brigade.
“The man, thought to be aged in his 50s, has been taken to hospital; his injuries are being treated as life-changing.”
London Fire Brigade said it sent six fire engines to the scene, following reports a tree had fallen on a pavement.
A spokesman said: “We were called to the incident at 1.37pm. Initial reports said that a pedestrian was trapped but on arrival this was not the case.
“The path was cleared and the incident for LFB ended at 1.42pm.”
Bus routes in the area were also disrupted by the fallen tree.
Transport for London Buses tweeted: “Routes 60 and 118 are diverted via Streatham High Road, London Road and Stanford Way due to an emergency services incident on Streatham Vale.
“Bus stops on Greyhound Lane, Streatham Vale and Rowan Road are not served.”
The news came as services were suspended between West Ham and Limehouse after a tree caused damage to the overhead wires.
Trees on the lines also caused delays between Ashford International and Hastings, Guildford and Reading, Newbury and Westbury and between Ipswich and Lowestoft on Saturday morning.
All the trees have since been cleared, National Rail said, although passengers may still experience disruption.
Network Rail has urged passengers to check for updates before they travel.
Across the rest of the country, the weather is expected to be much more settled on Sunday with showers and some patches of sunshine coming through.