Explosions and gunshots injured 28 people at a rally in central Bangkok Sunday, raising tensions as demonstrators prepared to launch a second week of "shutdown" protests aimed at toppling the Thai government.

The injured were taken to several city hospitals after the attack, the latest in a series by unknown assailants on the demonstrators.

"Seven of them are seriously injured... it's likely to be from shrapnel. Most of them are undergoing operations," Suphan Srithamma, the head of the Department of Medical Services, told AFP.

The capital's Erawan emergency centre confirmed the toll from two blasts followed by gunshots at Victory Monument, a major intersection held by the protesters since Monday.

Protesters said the injuries were caused by blasts minutes apart.

"People were speaking on stage when there was a loud explosion," said Theerayuth Utakaintanond, 52.

"Everybody hit the floor. Almost two minutes later, I think, I heard another explosion and everyone started to run in panic."

An AFP reporter saw blood-splattered clothes and a small crater at the scene of one of the blasts.

Twin explosions hit Thailand protests
A Thai bomb squad unit examines the site of a blast at Victory monument in Bangkok on January 19, 2014. --AFP PHOTO/ Nicolas ASFOURI

Thailand has been rocked by bouts of bloody unrest since just before a 2006 military coup that ousted Prime Minister Thaksin Shinawatra. He now lives in self-exile overseas to avoid a jail term for corruption.

His younger sister, Prime Minister Yingluck Shinawatra, has clung on to power through more than two months of street rallies aimed at forcing her elected government from office and installing an unelected "people's council".

She has called elections for February 2, despite a vow by the main opposition party to boycott them and threats from protesters to disrupt the polls.

The demonstrators have staged a self-styled "shutdown" of the city since last Monday, erecting roadblocks and rally stages at several intersections including those in its commercial heart.

The action has brought inconvenience to the city of 12 million, but authorities say the numbers on the streets appear to be dwindling -- although tens of thousands join nightly rallies.

The demonstrators are urging the military and independent institutions to bolster their attempt to block the election, which Yingluck is again expected to win.

On Friday one protester died and dozens were injured in a blast at an anti-government march in the city led by firebrand protest leader Suthep Thaugsuban.

Tensions in Bangkok soared following that blast, but demonstrators have vowed to continue their rallies.

"Suthep is not tired, he is not afraid," his stepson and rally spokesman Akanat Promphan said before Sunday's incident.

"I think now the protesters have two choices -- to go back home with nothing or to continue."

Police on Sunday said they were hunting two suspects shown in television footage leaving the scene of Friday's attack.

The current wave of protesters is made up of a coalition of Thaksin's foes among the Bangkok middle class, southerners and the pro-royalist elite.

But the ousted leader has strong support in the north and northeast, which has helped him or his allies win every election in Thailand this century.