TAIPEI, April 28 -- An air defense drill involving all cities and counties in northern parts of Taiwan stopped traffic in populated areas for a half an hour on Tuesday.

All vehicles except emergency vehicles were ordered to pull over. Pedestrians were told to evacuate and stay indoors or in shelters where available from 1:30 to 2:00 p.m. Tuesday.

The Wan An No. 38 exercise covered all parts of Taipei City, New Taipei City, Taoyuan City, Keelung City, Yilan County, Hsinchu County and Hsinchu City.

Air defense sirens were heard in Taipei but air traffic was not disrupted, according to Taiwan authorities.

Railways and mass rapid transit systems in the seven cities and counties in northern Taiwan continued to operate.