Several more were wounded in Friday's attack, according to Abdifatah Omar Halane, the spokesman for the Mogadishu municipality, adding that the death toll could rise significantly.
Somali group al-Shabab claimed responsibility for the attack, according to pro-insurgent radio Andalus.
Al-Shabab gunmen stormed the hotel in central Mogadishu after detonating a car bomb outside its gates.
At least five militants and two security personnel were killed in the attacks, according to security official Mohamed Hassan.
It wasn't immediately clear if their deaths were included in the official toll of 20.
There was ongoing gunfire at the SYL hotel, home to government officials, business professionals and foreign nationals.
It is located near the heavily secured presidential palace.
Last year, al-Shabab targeted the same hotel in a suicide car bombing as Turkish delegates were preparing for the visit of President Recep Tayyip Erdogan.
Fifteen people, all of them Somalis, were killed.
A second blast, believed to also have been a car bomb, went off in the same area a short time after the first explosion, senior Somali military official Abdullahi Madobe said.
At least eight people have been taken to hospital, an official at Madina Hospital said.
The first blast was massive and is thought to have caused many civilian casualties, also impacting close to 25 buildings in the area, including homes, restaurants and hotels.
Several bodies were believed to be under the rubble and rescue teams were already at work in the area.
Militants from al-Shabab, which is seeking an Islamist state in Somalia and is affiliated with the international al-Qaeda terrorist network, attack government facilities in Mogadishu on a regular basis.
