The cards distributed by Tatsyana Ravyaka and Uladzimer Labkovich also contained information about Byalyatski's 2011 trial.
The two managed to inform RFE/RL by phone about their arrest.
Byalyatski, who is the founder of Vyasna, also helped found Belarus's opposition Popular Front.
He was sentenced in 2011 to 4 1/2 years on tax-evasion charges that his supporters say were politically motivated.
The charges stemmed from Byalyatski's alleged use of personal accounts in Lithuania and Poland to receive funding from international donors for human rights activism in Belarus.
The U.S. State Department spokesman Marie Harf called on the Belarusian authorities on August 3 to immediately release Byalyatski and other political prisoners in the country.
Amnesty International has declared Byalyatski a prisoner of conscience.