Guinea's government called for calm on Wednesday after police fired teargas at protesters in Conakry, the capital, before provisional election results are announced later this week.
Millions of voters, around 75 percent of the population, cast their votes on Sunday, in the West African country's second free election in nearly 60 years since independence.
The election was calm and won praise from international observers, but tensions have since been mounting amid allegations of fraud by President Alpha Conde's challengers.
Guinea has a history of political violence, including protests during the 2010 campaign that brought Conde to power. On the Friday before this year's election, at least two people were killed and 33 injured in fighting between supporters of Conde and his main rival, Celloun Dalein Diallo.
On Wednesday, anti-riot police in the suburb of Koloma Soloprimo fired teargas and warning shots as protesters began building street barriers, residents said.
"It is heating up over here. We are all hiding in our houses," said resident Souleymane Bah.
A Reuters reporter on Monday saw three people with gunshot wounds at a local clinic after security forces and the opposition clashed overnight. One of the injured, Bachir Barry, said he was hit in the hip as he was walking from the market.
"We are calling on everyone to give up on the street (protests). If the institutions are not respected, then there is no rule of law," Foreign Minister François Lonseny Fall said at a meeting attended by press and foreign diplomats on Wednesday.
Justice Minister Cheick Sako said at the same meeting that those caught protesting would face criminal charges.
Dozens of anti-riot police vehicles circulated in opposition neighbourhoods, where burnt-out tyres and rocks littered the streets following the clashes late on Tuesday.
Results of the polls were due to be released on Wednesday, but the Independent National Electoral Commission has indicated they will be come later this week.
Early radio announcements show Conde with a sizeable lead. Most analysts, though, expect results to be close enough to require a second round, most likely against Diallo.