Polling stations opened across Tajikistan at 6 a.m. local time (0100 GMT) on Sunday and were about to close at 8 p.m. (1500 GMT), with results scheduled to be released the next day.
Rahmon, who is running for a fifth term, has been at the helm of the 9.5-million-strong country for nearly three decades and hailed as a figure that brought peace and stability since he led his nation out of civil war in the 1990s.
Voters in the capital Dushanbe overwhelmingly stated their intention to cast their ballots for Rahmon, AFP reported. The 68-year-old faces four other candidates, all of whom are viewed as token opponents in the Sunday vote.
The Social Democratic party, the biggest opposition force, has boycotted the election in protest at constitutional rules which it says tilt the playing field to ensure the dominance of Rahmon's People's Democratic Party.
Constitutional changes passed in 2016 paved the way for Rahmon to run for office for an unlimited number of times, putting him on course to become the longest-ruling leader among those of ex-Soviet states after Kazakhstan's Nursultan Nazarbayev, who retired last year after 29 years in power.
Analysts believe that any future succession to presidency is likely to be hereditary in Tajikistan after Rahmon's son, Rustam Emomali, was elected chairman of parliament's upper house earlier this year, in a move that positions him as constitutionally second-in-line to the presidency.
The Tajik leader is more inclined to Moscow and his country has close economic ties with Russia. Hundreds of thousands of Tajiks are working in Russia to support families at home.