Ten suspects have so far been detained by the Belgian authorities on terrorism charges connected with the attacks.
Prime Minister Binali Yildirim said 7,543 people had so far been detained, including 6,038 soldiers.
Another senior administration official said that while there had been no need so far to detain ailing passengers arriving at American airports from overseas, the government feared that the day would come soon when large numbers might need to be stopped.
As is customary for Eta prisoners, the woman's son and his wife were detained far from home.
His company runs the notorious Yarl's Wood detention centre where children have been detained far beyond the 28-day with charge maximum allowed for terror suspects.
It was unclear how prosecutors linked them to the May 6 violence — Ms. Sobchak did not attend the event at all, and Mr. Navalny and Mr. Udaltsov were detained far from the clashes, as they prepared to speak from a podium.
"He was detained far longer than he should have been legally held," he said, according to The A.P. "The defense lawyer was not even allowed to meet the witnesses until a night before the trial".
By the end of the day, as many as a thousand people in Moscow had been detained, far more than in 2011 and 2012, the last time the capital saw demonstrations of this size.
Both major parties effectively argue that the ends justify the means, that "stopping the boats" and "preventing the drownings at sea" justify the collateral damage – the men, women and children detained far away with no idea when, if ever, they will be able to build a new life.
The animal rights group's advocacy efforts made it a target of protester chants, and so far, police have detained about 200 people since protests began three days ago.
