Russian police have detained more than 200 people near a Moscow court which is considering whether to jail Kremlin critic Alexei Navalny, monitors say.
Many riot police, including some on horses, are deployed outside the court.
Mr Navalny said in court the charges against him were fabricated and again accused President Vladimir Putin of trying to poison him.
The hearing is to decide whether to turn Mr Navalny's suspended sentence into an actual prison term.
He could face up to three-and-a-half years, in a case that has sparked nationwide protests.
The arrests were reported by the Russian OVD-Info monitoring group, which documents police activities, and by the Moscow Public Monitoring Commission (ONK), a human rights body.
Mr Navalny's return to Russia on 17 January triggered mass protests in support of him, many of them young Russians who have only ever experienced President Putin's rule.
Mr Navalny has been accused of breaking the terms of a 2014 suspended sentence for embezzlement that required him to report regularly to Russian police. His lawyers say the accusation is absurd as the authorities knew he was recovering in Berlin after a nerve agent attack that nearly killed him.
Addressing the court on Tuesday, Mr Navalny said the case was being used to frighten the opposition: "The main thing in this process is to intimidate a huge number of people. This is how it works: they send one to jail to intimidate millions."
He also addressed the nerve agent attack in August: "Using the FSB [Federal Security Service of Russia], Putin attempted to commit murder. I'm not the only one - many know this already and many others will. And this is driving the thieving little man in the bunker crazy.
"No matter how much he tries to look like a geopolitician, he took offence at me because he will go down in history as a poisoner."
Western diplomats are attending the hearing. The EU has condemned the mass arrests of Navalny supporters, and EU foreign policy chief Josep Borrell is to visit Moscow on Thursday for official talks.
Commenting on the court hearing, Kremlin spokesman Dmitry Peskov said "we hope that such nonsense as linking the prospects of Russia-EU relations with the resident of a detention centre will not happen".
Just before the court hearing began, Mr Navalny praised his wife Yulia, who is attending in court. She was fined 20,000 roubles (£190; $260) on Monday for having joined the pro-Navalny protesters at an "unauthorised" rally.
"They said that you had seriously violated public order and were a bad girl. I'm proud of you," Mr Navalny said, quoted by Reuters news agency.
In Room 635 of the Moscow City Courthouse, four police officers have been guarding a glass cage. Locked inside is President Putin's most ferocious critic.
For much of the time Alexei Navalny has been following proceedings in silence: standing with his arms folded. Sometimes he has interjected.
"Why are you deceiving the court by saying you didn't know where I was?" Mr Navalny responded when the Moscow Prison Service claimed he had gone into hiding in Germany. "You had my address, my contact details."
Several times the prosecutor has interrupted the defence lawyers, complaining about their line of questioning.
On the courtroom wall above me is a portrait of a famous Russian judge from tsarist times, along with the quotation: "Words are one of the greatest weapons of man."
After his "Putin's Palace" video notched up more than 100 million views online, Alexei Navalny may well agree.