A new plasma source, the so-called diffuse coplanar surface barrier discharge (DCSBD), is described. DCSBD allows a visually diffuse high-density 'cold' plasma to be sustained in atmospheric-pressure air at a high plasma power density exceeding 100 W cm−3 that permits high-speed surface processing of large-area webs and flat surfaces. This is demonstrated by the results on a successful in-line activation of thin polypropylene fabric at 450 m min−1 and plasma exposures as short as 0.14 s. DCSBD basic features resulting in the observed high efficiency of plasma activation and the related plasmachemical mechanism are discussed briefly.