RIO DE JANEIRO — A movement that has been gathering steam at the Olympic Aquatics Stadium was ready for its hashtag on Monday night. The Clean Athletes Matter brigade found a ready, able and ardent leader: Lilly King of the United States, who entered the Rio Games with the top time in the world in the women’s 100-meter breaststroke.

The next fastest swimmer in the lead-up to the competition was Yulia Efimova, a 24-year-old Russian whose place at these Olympics was not finalized until Saturday. Efimova, the reigning world champion, served a 16-month doping suspension handed down by FINA, swimming’s global governing body, which ended in early 2015.

Early this year, Efimova failed a test for meldonium, but the result was overturned when she appealed to the Court of Arbitration for Sport, arguing that she already had served her penalty.