HSBC's composite emerging markets index of manufacturing and services purchasing managers' surveys edged up to 50.4 from 50.3 in March, but remained well below its long-run trend level of 53.9.
The data showed declining output in the four largest economies, Brazil, Russia, India and China - below the 50 threshold that marks the difference between expansion and contraction.
Based on data from purchasing managers at about 8,000 firms in 17 countries, the survey showed manufacturing output across emerging markets was broadly stagnant in April, while services activity growth was unchanged from March's weak rate.
"Big economies from Russia, Turkey, Poland to South Africa all lost momentum," said Murat Ulgen, HSBC chief economist for central and eastern Europe and sub-Saharan Africa, in a statement.
The future output index, which tracks firms' expectations for activity in 12 months' time, fell to a new low in April, mainly reflecting a sharp weakening in output expectations in Brazil and Mexico and the weakest sentiment in China in 2014 so far.
The HSBC index is calculated using data produced by Markit.