“Some members of the security forces positioned themselves just at the entrance to the place where we were meeting,” said opposition politician Nathaniel Olympio.
“Very menacingly, they started to throw teargas everywhere. We had to leave the premises after 45 minutes,” he told AFP.
Videos of indiscriminate beatings of protesters by security forces emerged on social media on Saturday.
The 14-party opposition coalition, which has staged anti-government protests since last August, this week decided to resume demonstrations, despite an official ban.
But since Wednesday, security forces have closed off all opposition meeting points in the capital. The teargas on Saturday was fired near the opposition headquarters.
There was also unrest in Kpalime, some 120 kilometres (75 miles) north of Lome, where strong action had been taken to prevent demonstrators taking to the streets.
The opposition is calling for a return to the 1992 constitution, which imposes a two-term limit on presidents, and also the introduction of two-round voting in elections.
Gnassingbe has been president since 2005, taking over after the death of his father, General Gnassingbe Eyadema, who ruled Togo for 37 years.
His son won elections in 2005, 2010 and 2015. The opposition wants the application of the two-term limit to be retroactive to prevent him standing again.
A sticking point to agreement has been the government’s refusal to apply the retrospective element, raising the possibility Gnassingbe could remain president until 2030.
Talks brokered by Ghana’s President Nana Akufo-Addo and his Guinean counterpart Alpha Conde stalled earlier this year after just three sessions.
On Saturday, regional heads of state gathered in Lome for a meeting chaired by Gnassingbe, who holds the rotating presidency of the West African bloc ECOWAS.
In a statement issued after closed-door talks, the grouping called on Akufo-Addo and Conde to “intensify their efforts to help Togo’s government and political actors” towards reform.
It also called for an end to violence during the process.