The Reds will take on Real Madrid at the Stade de France, just north of the city, on Saturday evening.
As many as 50,000 supporters, largely ticketless, began gathering early in the day in different parts of the city, but police reminded them to be wary.
Merseyside Police’s Liverpool matchday policing Twitter account posted: “Just been made aware of an attempted theft up at the stadium.
“What appears to be local youths snatching off people and running off.
“Look after you belongings. Look out for each other. Stay safe.”
Liverpool great Kenny Dalglish, UFC fighter Paddy Pimblett and musician Ian Broudie joined a huge sea of red shirts in the Cours de Vincennes area in the south east of the city, where a fan zone has been set up.
Pimblett was greeting fans in a Liverpool hat, while Broudie performed in front of the crowd.
Footballs were kicked in the air and banners flown while flares – which were prohibited by French authorities – were let off in the fan zone.
Some fans were hanging high above the crowd on lampposts, letting off flares.
The huge crowd then sang the club’s unofficial anthem You’ll Never Walk Alone before cheering Dalglish to a stage.
He told the crowd how remarkable Liverpool’s season had been before fans chanted his name.
A heavy police presence is in the area, including armed officers.
Supporter Ricky White, 45, from Aintree, told the PA news agency: “I’m nervous now, hearing King Kenny speak, seeing the fans, I really hope we do it.
“This is a special club which deserves success, I just hope it happens.”
John Racks, 39, from the Wirral, said: “Yeah, the drinking started early, the sun’s out now and it’s getting to my head a bit.
“I’d like to say at nearly 40 I’m careful but I kind of got lost and started drinking this morning, now I’m smashed.
“The atmosphere here is boss, the city is red.”
Another said: “I’m sat here on top of some statue in Paris with my mates, there’s nothing more you need. Just a few beers and hopefully a win for the Reds, lovely.”
Fans were carrying large boxes of beer and singing about team manager Jurgen Klopp and the players.
Luke Queens, 23, said: “I’m proper nervous – confident in the team but it’s a final isn’t it?
“I hope the beer settles my nerves before then, we need the first goal.”