At least 20 people were killed in Mogadishu after al-Shabaab extremists stormed the Somali capital’s main court buildings, firing guns and setting off explosions during clashes with security forces.

The assault was the most serious in the capital since the Islamist group was forced out in 2011.

The attack on the supreme court triggered a gunfight with police and armed forces. Two explosions were heard and gunmen, many wearing explosives-laden vests, climbed on to the roof and fired shots.

A police officer, Hassan Abdulahi, said he saw five bodies at the entrance to the court. All nine extremist were killed, he said, adding that the militants had taken an unknown number of hostages, while other government workers and civilians on the premises hid.

Soldiers carry a wounded civilian from the court. Photograph: Farah Abdi Warsameh/APHours after the siege, a suicide car bomber rammed a vehicle carrying Turkish citizens.

On Friday, the Foreign Office released a travel warning for Somalia that warned of a high threat of terrorism. “We continue to believe terrorists are in the final stages of planning attacks in Mogadishu,” it said.

On a Twitter feed believed to belong to the militants, al-Shabaab appeared to take responsibilty for the attack. A post said five militants from the “Martyrdom Brigade” took part in the “daring” raid.

The Somali prime minister, Abdi Farah Shirdon, said the “pointless and pathetic act” would have no effect on the country’s commitment to progress.

President Hassan Sheikh Mohamud said: “I want the terrorist to know that our country, Somalia, is moving, and will keep moving forward, and will not be prevented to achieve the ultimate noble goal, a peaceful and stable Somalia, by a few desperate terrorists.”

A car is engulfed by flames near the scene of the attack. Photograph: Feisal Omar/ReutersThe court complex is a confusing maze of buildings and rooms, allowing for plenty of places to hide but also for many places for gunmen to take hostages. The armed men forced their way inside the complex and immediately set off an explosion, said Yusuf Abdi, who was near the court when the attack happened.

About two hours after the assault began, survivors of the attack began coming out of the buildings. Some were crying and others held their head in their hands.

“I never expected to make it out alive today,” said Halima Geddi, who had taken cover behind a wall. “There is no peace. No one protects us. I came to see my boy who was supposed to be tried here.”

Most militant attacks in Mogadishu are blamed on fighters from Al-Shabaab, an al-Qaida-linked rebel group that ruled the city from 2006 until August 2011, when African Union and government forces pushed them out of the city.

Since then extremists have launched suicide bombings on the capital every few weeks. Despite those intermittent attacks, Mogadishu is generally considered more peaceful than most of the previous seven years.