His slogans - "We want someone new" and "We're fed up" - resulted in charges of malicious hooliganism and premeditated destruction of property. The last charge carries a possible prison sentence of seven to 12 years.
"To my great regret, no other possibilities for expressing one's opinions have remained in Belarus," defendant Artur Finkevich, an activist with the Youth Front, told the court.
He said he "wanted to even the chances of presidential candidates, since the opposition was deprived of access to the media."
Finkevich, 21, has been in custody since Jan. 30.
The authorities estimated the damage caused by the graffiti at 35.2 million Belarusian rubles (US$16,500, Ђ13,000).
Ten of Finkevich's supporters were detained Thursday as they rallied outside the court with about 30 others, carrying the activist's portrait and calling for an end to political repression in Belarus.
About 40 opposition activists are already behind bars, including leader Alexander Milinkevich, who was jailed last week for 15 days after leading a Minsk protest that attracted about 10,000 people.
Lukashenko - often described by Western countries as "Europe's last dictator" - won a third term in March 19 elections deemed fraudulent by the European Union and the United States. The elections inspired a series of unprecedented mass rallies, which resulted in mass detentions.
Deputy Ambassador of the Czech Republic to Russia, Lubos Vesely, was among 20 diplomats, who were expelled from the Russian Federation