A challenger to Russian President Vladimir Putin in next month's presidential vote, Rybkin surfaced in Ukraine on Tuesday, five days after he had gone missing, but refused to throw any light on how and why he disappeared.
"I have a right to devote two or three days to myself," Rybkin said.
"I came to Kiev to visit my friends. I switched off my mobile phone and never watched television," he said.
But on his arrival in Moscow's Sheremetyevo airport, he was more cryptic, refusing to elaborate on his disappearance.
"I am back as if from a round of difficult talks in Chechnya and I am glad to be back," Rybkin said.
"I have a right to devote two or three days to myself"
Russian presidential candidate
Asked if he had been detained, Rybkin said "it is hard to detain me, but there are good people in Kiev, and I am very grateful to them."
A strong critic of Putin, Rybkin had mysteriously gone missing since last Thursday, fuelling speculations on his safety.
There were rumours that Rybkin may have been kidnapped.
Rybkin was officially registered on Saturday as one of the seven candidates in the presidential vote next month.
Hours later his camp announced that the candidate had gone missing and his wife feared for his life.
Analysts give Rybkin little or no chance of winning the elections and the initial media reaction to his disappearance was that it could be a publicity stunt.