The Spanish capital's emergency services received hundreds of calls about the smell of burning, while a satellite image shows a column of smoke spreading from Portugal.
The smell of burning from Portugal's massive forest fires reached Madrid on Tuesday, Spanish emergency services reported.
"We have received 380 calls on Madrid-112 from citizens worried about the smell of burning and smoke: it is a fire in Portugal," the Madrid region's emergency services wrote on Twitter.
A satellite image released by the rescue services shows a column of smoke spreading over 400 kilometres, the distance between Portugal and the Spanish capital.
Both countries, hit by drought and heatwave, are battling large forest fires.
The Serra da Estrela blaze -- already the biggest of this summer in Portugal -- has burned around 24,000 hectares, according to the latest provisional figures.
The fire, which broke out on August 6 in the vicinity of Covilha in central Portugal, destroyed unique forest areas in a UNESCO-recognised natural park.
At least 24 people have been injured -- three of them seriously -- while 45 others have had to be evacuated as a precautionary measure since Monday.
While the civil protection service has been criticised for its operational management, Portuguese Interior Minister José Luis Carneiro pledged on Monday to launch an evaluation of the "structural causes" and the "method of fighting" the fires "once the Serra da Estrela fire is extinguished".
Portugal, which is experiencing an exceptional drought this year, had its hottest July in almost a century.
Since the beginning of the year, some 81,000 hectares have gone up in smoke -- the largest area since the deadly fires of 2017 that killed around 100 people, according to the Institute for the Conservation of Nature and Forests.
Meanwhile, in the southeastern Spanish province of Alicante, 10,000 hectares have already been burned.
A fire, which spread after lightning struck the Vall d'Ebo on Saturday evening, has still not been brought under control. More than 1,500 people have had to be evacuated, according to the authorities.
In the region of Aragon in Spain's northeast, where more than 6,000 hectares have burned, firefighters seem to have succeeded in tackling the fire.
In Spain, forest fires in 2022 were three times more devastating than in the whole of 2021, when 84,827 hectares went up in smoke.