The euro dropped 0.8 percent to $1.12 against the dollar at 1710 GMT following the ECB's latest monetary policy meeting, where the bank also said interest rates would remain at historic lows until at least the end of the year, as had been expected.
ECB chief Mario Draghi warned that the eurozone was "coming out of, and maybe we still are in a period of continued weakness and pervasive uncertainty".
Draghi pointed to "factors... mostly of external source", including "the threat of protectionism" and "geopolitical considerations".
Eurozone equities sunk lower after the announcement, with Frankfurt's DAX 30 index closing down 0.6 percent and the Paris CAC 40 slipping 0.4 percent.
London's benchmark FTSE 100 index meanwhile lost 0.5 percent in value.
Wall Street followed suit, with the Dow Jones Industrial Average dropping 0.8 percent in midday trading.
- 'Glass half-empty' -
Economist Marcel Fratzscher of Berlin-based think-tank DIW, "the ECB sent a surprisingly clear warning signal today."
"The eurozone economy is weakening noticeably and risks are rising."
Economist Florian Hense of Berenberg bank agreed, saying that "previously, the ECB may have seen the glass half-full. With today's moves, it switched viewing the glass as half-empty."
The ECB also announced it would renew its "TLTRO" scheme, which allows banks to get super-cheap loans, in a bid to help support the eurozone economy and keep credit flowing.
European bank stocks fell sharply on the news.
"Investors realised that this very low interest rate environment was destined to last longer than expected, which resulted in a sharp drop in bond yields, as well as creating a toxic environment for banks, whose value fell sharply," said Daniel Larrouturou, a senior executive at investment firm Diamant Bleu Gestion.
However similar drops followed the announcement of previous TLTRO schemes in September 2014 and June 2016, Frederic Rozier, an asset manager at Mirabaud France in Paris, told AFP.
- 'Dovish minded' -
Unveiling the updated forecasts, Draghi said the bank now expects the eurozone's economy to expand by just 1.1 percent this year, down from the previous estimate of 1.7 percent.
He also said inflation would slow to 1.2 percent this year, compared with the 1.6 percent previously forecast, pushing the ECB farther from its target of just under 2.0 percent.
The United States has also had a series of mixed economic reports, with the Department of Labor reporting on Thursday that weekly jobless claims dipped by 3,000 to 223,000.
The report comes ahead of Friday's key jobs report for February. Analysts expect the US added 173,000 jobs last month and that the unemployment rate dipped to 3.8 percent from 4.0 percent.