He said the detention came soon after the deadly May violence in the eastern Uzbek city of Andijon and his receiving threats to stop working as a human-rights activist.
"I received the first threat before my release, in prison," Ustaboyev said. "They told me, they warned me. They said: 'Sobitkhon, you know very well yourself, that the 15-day detention is not the end. The prosecutor can revise the decision as new proofs of your guilt are found. The [criminal] case can be re-launched.' Three days before my release they told me: 'You ruined the fate of your eldest son. Now, think of the fate of your young son.' That was enough for me."
Ustaboyev's eldest son, along with his father, was involved in monitoring the human-rights situation in Namangan and the nearby city of Andijon where the violent clashes between protestors and government troops that erupted on 13 May led to the death of hundreds of civilians.
Rights activists say the number of victims may be as high as 1,000 and include many women and children. Uzbek authorities put the death toll at 187, saying many of them were terrorists and extremists.
Many human-rights activists, as well as independent journalists, have faced harassment and persecution after the Andijon events.
Kazakhstan became a possible destination for Uzbek refugees after Lutfullo Shamsutdinov, a human rights activist from Andijon, fled there shortly after the crackdown in eastern Uzbekistan.
Shamsutdinov had monitored the trial of the 23 businessmen accused of belonging to the banned Islamic group Akramiya that led to the Andijon uprising.
Shamsutdinov was arrested in Kazakhstan in early June at the request of Uzbek authorities who demanded his extradition, saying he was a suspected terrorist. But Shamsutdinov, together with six members of his family, were recognized as refugees by the UN Refugee Agency, or UNHCR.
The UNHCR put pressure on Kazakh authorities, saying they should not obey an extradition request from Uzbekistan because the 1951 Geneva Conventions forbid the forced return of refugees and asylum seekers to their country of origin.
Shamsutdinov was then released and flown to a European country where he received political asylum.
Rizokhoja Obidov, an independent journalist from Andijon, also fled to Kazakhstan, after initially being detained by Uzbek authorities following the violence in Andijon. He told RFE/RL he was encouraged to do so by the Kazakh authorities' decision not to collaborate with Uzbek security services.
"On June 25, at around one o'clock in the night, a group of policemen entered my apartment, and took me away," Obidov said. "They brought me to the Interior Ministry's Namangan City Department and interrogated me there for several hours, asking me if I had relatives in Andijon, then let me go home. But on 5 July, police again detained me and kept me in a police station for three days. Namangan's Deputy Prosecutor-General Ravshan Nazarov officially told me that I was accused of insulting the dignity and honor of the Uzbek president, according to Article 158 of the Uzbek Criminal Code. Then he questioned me and asked what I meant in my poem."
Obidov had written a poem devoted to victims of the Andijon tragedy, but he insisted there is nothing about the Uzbek president in it.
He first recited his poem on RFE/RL's Uzbek Service in May:
Blood was shed in Andijon. Who needed it?
Who was the one who needed the lives of mothers and children?
Who was that who gave the order to start 'steel rain'?
How can we believe those whose hands are stained with innocent blood?
Obidov has also received refugee status and now awaits his transfer to a Western country.
He said he and Shamsutdinov escaped the fate of another human-rights activist from Andijon, Saidjahon Zaynabidinov, who was arrested shortly after 13 May.
Zaynabiddionv defended the 23 businessmen in the Andijon court and became well known during the uprising as he gave numerous interviews to international media outlets.
A spokesman for Uzbekistan's national-security service, Olimjon Turakulov, said at the time that Zaynabiddinov was involved in planning of violence in Andijon on 13 May.
Zaynabidinov's fate remains unknown.
Obidov said the Uzbek government's crackdown on dissent became country-wide since the Andijon unrest.
"At present, there is a campaign against those in opposition, those who think freely, those who witnessed the Andijon events, or those who expresses their discontent with the government's reaction toward the Andijon protest and shooting of people," Obidov said. "There are repressions, not only of me, but many others."
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