JCDecaux, the world's biggest out-of-home advertising company, has reported a 63.4% decline in organic revenue for the second quarter of the year.
Sales in the period between April and June were €351.9m (£314m), compared with €1bn in the same period last year.
The slump was expected because of the coronavirus lockdown in many major markets, but the scale of the decline illustrates the impact of the ad downturn on the OOH sector, which has been one of the worst hit after cinema.
Across the first six months of 2020, organic revenue dropped 41.6% to €1.1bn.
In the UK, where JCDecaux's assets include Transport for London's bus-shelter contract, revenue for the first half fell to €98.5m, down 42.7% year on year.
Jean-Charles Decaux, chairman of the executive board and co-chief executive of JCDecaux, said: "During the Covid-19 lockdown period, the temporary historic drop in urban and transport audiences, as well as severe economic uncertainties, led companies to react immediately and to reduce their advertising spend in an unprecedented scale.
"Once lockdown measures were lifted, urban audiences started to recover progressively in street furniture and in billboard, while transport audiences are still lagging significantly, mainly in airports.
"Advertising revenue has, for the time being, not followed the same pace of recovery and we see an important difference between audience levels, which are in some geographies close to pre-Covid-19, and revenue levels, which do not yet reflect the positive momentum in urban audiences."
Decaux said the group has carried out "severe cost management" and he praised staff for their "commitment and solidarity, including salary cuts".
Like many other advertising companies, JCDecaux said it could not offer financial guidance for the remainder of 2020 because of the continued uncertainty.