File Photo: People walk past the wreckage of a vehicle destroyed in an attack on police and checkpoints on the outskirts of the capital Mogadishu, Somalia Wednesday, Feb. 16, 2022. AP
The attack was claimed by the Al-Shabaab Islamist militant group, which has been waging an insurgency in the troubled Horn of Africa nation for years.
Security had been tightened in Beledweyne ahead of a first session of voting for parliamentary seats in the constituency, which lies about 340 kilometres (210 miles) north of the capital Mogadishu.
"The number of people who have died in the heinous terrorist attack in Beledweyne today has increased from 10 people to 14 as of now," local police officer Mohamud Hassan told AFP by phone.
He said some of the 16 civilians earlier reported wounded in the suicide bombing had died of their injuries in hospital. Among the dead were local government officials.
"This was the deadliest attack I can recall in this town," he added.
Al-Shabaab said in a statement that one of its fighters carried out the bombing.
Somalia, particularly Mogadishu, has seen a spate of attacks in recent weeks as the country hobbles through a long-delayed election process.
Witnesses said the huge explosion tore through an open area of the Hassan Dhiif restaurant where people had gathered under trees to eat lunch.
"I saw dead bodies of several people and I could not count how many wounded that were rushed to hospital," said one witness, Mahad Osman.
"Some of these people were waiting for their ordered meals to come while enjoying the fresh weather when the blast occurred," he added.
"I saw... shoes, sticks and hats strewn at the scene of the blast, there was also blood and severed parts of human flesh in the area."
In another incident on Saturday, one person was killed and six others wounded when an explosive device went off in a teashop in Bosaso, the commercial capital of the northern state of Puntland, police said.
No-one has yet claimed responsibility for that blast.
Somalia is due to wrap up voting for the lower house of parliament by February 25 under the latest timetable for the elections, which are already more than a year behind schedule.
President President Mohamed Abdullahi Mohamed, better known by his nickname Farmajo, has been at loggerheads with Prime Minister Mohamed Hussein Roble over the election delays, an impasse that has Somalia's international backers worried.
Among those running for a seat in Beledweyne is Farhad Yasin, Somalia's former intelligence chief who is now Farmajo's national security adviser.
Somalia's voting process follows a complex indirect model, whereby state legislatures and clan delegates pick lawmakers for the national parliament, who in turn choose the president.
Voting for the upper house concluded last year, while clan delegates have so far elected 159 of the 275 MPs who sit in the lower house.
Somalia's international partners fear the election crisis distracts from the battle against Al-Shabaab, the Al-Qaeda-linked Islamist group fighting the weak central government.
Its fighters were driven out of Mogadishu in 2011 after an offensive by an African Union force, but they still control vast swathes of rural Somalia from where they launch regular attacks in the capital and elsewhere.
The United States issued a statement on Friday calling on Somalia's leaders to complete the elections in a "credible and transparent manner" by February 25.
"The United States will hold accountable those who obstruct or undermine the process," it said.
Earlier this month, US Secretary of State Antony Blinken announced a decision to restrict visas to current or former Somali officials or others "believed to be responsible for, or complicit in, undermining the democratic process in Somalia".