Omnicom's global organic revenue plummeted 23% in the second quarter of the year, showing the major impact that the coronavirus pandemic has had on the business.
The company, which owns the DDB and BBDO ad networks, reported revenue of $2.8bn (£2.4bn) for the three months to 30 June, down 25% year on year on a reported basis.
Operating profit plunged 89% to $62.5m, compared with $573.7m in the same period last year.
The world's second-biggest agency group ran up some one-off costs, including $150m in severance costs, as it cut 6,100 jobs.
The group previously said it employed about 70,000 people at the end of 2019.
Omnicom's advertising business reported a 27% fall in organic revenue; CRM consumer experience showed a 26% decline; CRM execution and support decreased 28%; and PR fell by 13.9%.
Much like its competitors, Omnicom reported an uplift in its healthcare division by 3.2%.
In the UK, where the business owns Adam & Eve/DDB, Abbott Mead Vickers BBDO and OMD, there was a 24% drop in organic revenue.
Omnicom said the reduction in revenue is expected to continue for the rest of the year.
The report said: "While we have a diversified portfolio of service offerings, clients and geographies, demand for our services can be expected to decline as marketers reduce expenditures in the short term due to the uncertain impact of the pandemic on the global economy.
"During the second quarter of 2020, we realigned our agencies' cost structures, which included severance actions and furloughs to reduce the workforce, right-of-use asset impairments and other real estate costs, a net loss on the disposition of certain subsidiaries and other charges.
"These actions were taken to tailor their services and capabilities to changes in client demand."
Omnicom's 23% organic revenue decline was significantly worse than French rival Publicis Groupe, which reported a 13% drop in the second quarter.
Ian Whittaker, a long-time media analyst in London, said the results were "much worse than Publicis' surprise positive news last week".
He noted Omnicom's performance was particularly weak in North America and Asia-Pacific compared with the French group.
"It suggests the performance of the big global agency groups may be more varied than expected," Whittaker said.
Analysts at Barclays said the results were a "surprise on the downside" and the outlook for the rest of 2020 was "also surprisingly downbeat".