Counting 1 million crowds at anti-president rallies in Seoul

South Korea is seeing its biggest wave of street demonstrations in decades, but nobody is sure how many people are taking to the streets each week.

For the fifth straight weekend, masses of protesters occupied major avenues in downtown Seoul on Saturday demanding the ouster of President Park Geun-hye. She is suspected of helping in the criminal activities of a secretive confidante who is accused of manipulating government affairs and extorting companies to build an illicit fortune.

The rally renewed what has become a weekly back-and-forth between police and protest organizers, whose crowd estimates have differed widely. Police said about 270,000 people turned out on...