The opposition leaders were arrested on Saturday, hours after hundreds of anti-government protesters
gathered in Almaty, Kazakhstan's largest city, for a rare demonstration calling for democratic change.
Bulat Abilov, co-chairman of the All-National Social Democratic Party, or OSDP, had used the rally to demand a transparent investigation into riots last month in the oil-producing region of Zhanaozen, the Central Asian state's
deadliest violence in decades.
An Almaty court later sentenced Abilov to 18 days in jail, while handing Amirzhan Kosanov, the OSDP deputy chairman, and Amirbek Togusov, the party's Almaty head, 15 days in jail each.
The three were sentenced on charges of violating the law on holding protests.
"What we are doing is telling the country the truth, we are fighting for honest elections,'' Kosanov told The Associated Press news agency by telephone. "This punishment will not change our position.''
Al Jazeera's Robin Forestier-Walker, reporting from Almaty, said the relatively small number of people who turned up at the protest gives an indication of how "limited open dissent towards the government here is".
"If there are many more who are unhappy about the way things are going, they are not prepared to stand up and be counted," he said.
The rally marked the second peaceful protest since a January 15 parliamentary election
gave Nazarbayev's Nur Otan party an overwhelming victory, with 83 of the parliament's 107 seats.
After denouncing the election as rigged and faulty during an unauthorised rally on January 17, Abilov and Kosanov were fined and warned they could be arrested next time.
Nazarbayev has ruled Kazakhstan since before independence with little tolerance for dissent.
This month's election admitted three parties to parliament for the first time, but observers from the Organisation for Security and Co-operation in Europe (OSCE) said it lacked any genuine opposition presence.
"The election wasn't legitimate. We want them to hear us," Ravilya, a pensioner who stood in the crowd in temperatures
of minus 10 degrees Celsius, told the Reuters news agency.
"There are more police than people. It's a good thing they're armed only with sticks," she said.
Source: Al Jazeera and agencies