At least 12 guards were killed, with some 20 more wounded in the detonation on a busy street in the Tunisian capital late Tuesday.

"I saw the bus blow up. There were bodies and blood everywhere," witness Bassem Trifi told Reuters.

The guardsmen were boarding the vehicle which was meant to take them to the presidential palace on the outskirts of the city for the start of their shift.

The Tunisian president Beji Caid Essebsi called for international cooperation against the extremists, and vowed that his government would "vanquish terrorism" in a televised address.

"As a result of this painful event, this great tragedy... I proclaim a state of emergency for 30 days under the terms of law, and a curfew in greater Tunis […] until 5:00 am (0400 UTC) tomorrow," he said.

The president also cancelled a trip to Europe he had scheduled for Wednesday.

The blast comes ten days after authorities raised the security level and deployed additional security forces in the capital, citing information about possible terror attacks. Earlier this month, the officials arrested 17 Islamist militants, claiming they had prevented a large assault.

Democracy under attack

The Tuesday blast is the third large terror strike to hit the African country since the beginning of the year.

A shooter killed 38 people, mostly foreign tourists, at a luxury beach hotel in Sousse in June. A separate attack at the Bardo Museum in Tunisia killed another 21 tourists in March.

The "Islamic State" claimed responsibility for both of those attacks.

Tunisia is often praised as a rare Arab Spring success story, with various political parties forming a broad democratic consensus.

However, it has come under increasing pressure from militants groups, and thousands of its citizens are fighting for the Islamists in Syria, Iraq and Libya.

dj/bw (Reuters, AP, dpa, AFP)