Supporters without tickets for Wales' Euro 2016 game against England should stay away, warned the head of a police delegation heading to France.
Supt Steve Furnham said infrastructure in Lens is "very limited" and fans should watch the game in other cities.
Welsh officers will join colleagues from England and Northern Ireland as 500,000 UK nationals head to France.
Supt Furnham also said banned fans will get a "knock on the door" if they do not surrender their passports.
A police delegation from Wales travels to Sweden with the national team for a friendly on 5 June and then on to Paris the next day.
"We are there until Wales get knocked out or win it. Hopefully the latter," South Wales Police's Supt Furnham said.
Their role will be to patrol host cities and fan zones as well as offering guidance to French police on safety, security and how to engage with Welsh fans.
Supt Furnham said officers chosen from Wales are "very experienced" who regularly work at domestic games as well as at internationals.
A total of 87 Welsh fans issued with banning orders after incidents in the UK have until Wednesday night to surrender their passports.
"The very fact they have been arrested indicates they are a risk," he said.
"We've worked really, really hard to reduce the number of arrests at games across the country. Arrests are down and we want that to continue."
With a fan zone capable of holding just 10,000 fans, a handful of bars and a ban on consuming alcohol on the streets of Lens, Supt Furnham warned ticketless Wales fans against going to the city for the game with England on 16 June.
Lens has a population of less than 40,000 and with the stadium holding just 35,000, concerns have been expressed about a large influx.
"If you haven't got a ticket to Lens, don't go," he said.
"It's been well documented in the last couple of weeks that the infrastructure doesn't lend itself to large numbers of football fans going there."
He said other cities are better equipped such as Bordeaux, where there will be a fan zone capable of hosting 60,000 people.