Navalny's comments were piped into a hearing in a Moscow courtroom, where he lost his appeal against a fine for defaming a World War 2 veteran.
He faces further legal pressure, with his team saying he was hit with new criminal charges. Allies were forced to disband his network of regional campaign offices, which the authorities are seeking to ban as "extremist".
His head shaven, Navalny said he had been taken to a bathhouse to look "decent" before the court hearing. He undid his prison uniform to reveal a T-shirt that barely hid his thin torso.
"I looked in the mirror. Of course, I'm just a dreadful skeleton," he said. One of his lawyers said he had lost 22kg since January.
Later in the hearing, Navalny (44) went on the attack against Putin and the Russian justice system.
"I want to tell the dear court that your king is naked," he said of Putin. "Your naked, thieving king wants to continue to rule until the end ... Another 10 years will come, a stolen decade will come."
Addressing his wife Julia, who was in court, he said he missed her and asked her to stand so that he could look at her.
Describing how he was gradually ending his more than three-week hunger strike, he said he had eaten four spoonfuls of porridge on Wednesday. Requests for carrots and apples had not yet been granted.
'SWINDLERS AND THIEVES'
Navalny is serving a two-and-a-half year jail sentence for parole violations on an earlier embezzlement conviction that he says was politically motivated.
He declared his hunger strike on March 31 to demand better medical care for leg and back pain. On April 23 he said he would start eating again after getting more medical care. Russia has said he is receiving the same treatment as any other prisoner and accused him of exaggerating his health needs for publicity.
Navalny rose to prominence with an anti-corruption campaign of caustic videos cataloguing the wealth of senior officials he labelled "swindlers and thieves", and has become Putin's fiercest political rival.
A separate court is considering whether to declare Navalny's Anti-Corruption Foundation (FBK) and his network of regional campaign offices "extremist", which would give authorities the power to jail activists and freeze bank accounts. That court will hold its next hearing on May 17.
"Maintaining the work of Navalny's network of headquarters in its current form is impossible," Leonid Volkov, a Navalny ally, said in a YouTube video announcing the closure of the regional offices. Many will now function independently, he said.
Also speaking on a YouTube broadcast, the Anti-Corruption Foundation's head, Ivan Zhdanov, said the group may not exist as a legal entity after a court ruling in the extremism case, but its activities would not cease.
In Arkhangelsk, in northern Russia, a judge handed down a two-and-a-half year sentence on Thursday to the former coordinator of one of Navalny's campaign offices for reposting a music video which was deemed to contain pornography.
Navalny's allies also said a new criminal case had been opened against him for allegedly setting up a non-profit organisation that infringed on the rights of citizens. This could not immediately be confirmed.
Last year, Navalny survived an attack with a nerve agent. After recovering in Germany, he was arrested on his return to Russia in January and sentenced the following month.
A large mural of Navalny on a building in St Petersburg was painted over by Russian authorities on Wednesday.
Russian media said on Thursday police were investigating, and those behind it could face up to three years in prison for vandalism.
