Sixteen of the defendants are foreigners, including five Russians and six citizens of Turkey, the Russian news agency ITAR-Tass reported.
They face various charges, including attempted assassination, organising a crime network and plotting to overthrow the government.
President Niyazov's motorcade reportedly came under fire from machine guns during the attack on 25 November last year, but he was unharmed.
If convicted, many of the defendants could face up to 25 years in prison terms.
Mr Niyazov has said any foreigners found guilty of "indirect involvement" would be deported, according to ITAR-Tass.
Opponent jailed
Last month former foreign minister Boris Shikhmuradov was sentenced to life imprisonment for masterminding the attack.
Mr Shikhmuradov, who had been living in exile, was arrested inside the central Asian republic on 25 December and sentenced three days later.
In a confession shown on television, Mr Shikhmuradov said he had planned the assassination attempt while under the influence of drugs.
On Wednesday, his sister Larisa said Mr Shikhmuradov had a heart condition and she feared for his life.
Critics have likened the judicial process in Turkmenistan to the show trials conducted under Stalin in the former USSR in the 1930s.
Mr Niyazov, who has ruled the energy-rich country with an autocratic grip, has named three other exiled opponents as being behind the reported attack:
Turkmenistan is rife with political discontent, where government officials are frequently subject to arbitrary cabinet reshuffles and humiliating rebukes by the president, often broadcast live on television.
Mr Niyazov, 62, has been in power since before independence from the USSR in 1991 and styles himself the Father of all Turkmens.
He has constructed a personality cult around himself harking back to the days of Soviet dictator Joseph Stalin, and likewise tolerates no opposition to his rule.
Streets and towns have been given his name and a parliamentary decree has renamed the months of the year and days of the week after the president, his mother and Turkmen folklore heroes.
A giant golden statue of the president, revolving on top of a tower so that it always faces the sun, has been built in the centre of Ashgabat.
Turkmenistan has the fifth-largest deposits of natural gas in the world but most of its population live in abject poverty.