The Global Ozone Monitoring Experiment (GOME) routinely maps atmospheric column densities of chlorine dioxide (OClO), an important indicator of chlorine activation. In previous studies it was shown that the level of OClO during polar winters is anticorrelated to synoptic-scale temperatures inside the polar vortex. Here we present observations of exceptionally fast chlorine activation by mesoscale polar stratospheric clouds during the mountain wave event above northern Scandinavia between 20 and 22 January 1997. The observed slant column densities of OClO in the area of the mountain wave activity increase from 0.6 * 1014 molec/cm2 to 1.3 * 1014 molec/cm2. Combining the GOME observations with radiative transfer calculations, the corresponding vertical column densities are derived, and by applying pressure and temperature profiles from mesoscale simulations, the mixing ratios for OClO and ClO at the different altitude levels are estimated. The vertical column densities of OClO indicate a level of chlorine activation that is about two times higher compared to the days before the mountain wave event, and the deduced mixing ratios for ClO imply that chlorine was almost completely activated over a wide altitude range. The area showing enhanced OClO values increased by approximately 750.000 km2 which is more than 3% of the polar vortex area (≈23.0 * 106 km2). These observations are in good agreement with previous model studies, where comparable increases and areas of chlorine activation were calculated. The case of 21 January 1997 and similar GOME OClO observations in other Arctic winters confirm the importance of stratospheric mountain waves for the activation of chlorine species in the northern hemisphere.