It is believed the two men fled when officers were carrying out a raid in the area.
In a statement on Twitter, police said: “Large police presence in #Clapham High Street #Lambeth; two men scaled roof of property after warrant executed this morning.
“Both now detained, officers in process of bringing them down with assistance from @LondonFire. Road closures in place, please avoid area if poss.”
A spokesman for the Met Police confirmed to Express.co.uk that the two men have been arrested and the incident was currently being “brought to a conclusion”.
A police helicopter was also deployed to the area.
The incident occurred about a mile east of Clapham Junction railway station, close to Clapham Common.
Armed police surrounded a flat in the area, according to a witness.
Buses in the area have been disrupted with the 50 being forced to terminate early while routes 155 and 345 have been diverted due to the road closure.
A Met Police spokesman said: “Police executed a warrant at a residential address in Edgeley Lane, Clapham SW4 at around 07:02hrs on Saturday, 13 January.
“Two men are believed to have climbed onto the roof of the property.
“They have now been detained and officers are in the process of bringing them down. They will be taken to south London police stations.
“Following a search, two firearms were discovered at the property. Specialist firearms officers attended as a precaution.
“There have been no shots fired and no reported injuries.
“Colleagues from the National Police Air Service attended to provide support.
“Road closures have been put in place while the incident is dealt with.”
More to follow.