Bangkok: Thai authorities fear more attacks may be planned for the Thai capital after the second bomb attack in less than 24 hours.

National police spokesman said the second bomb used in an attack on Tuesday afternoon was similar to Monday evening's bomb at the Erawan shrine in central Bangkok.

Witnesses said an unidentified man threw a bomb from the Saphan Takin bridge over the Chao Phraya river onto a footbridge crowded with pedestrians.

But they said he missed the target and the bomb landed in the water and exploded.

No one was injured.

The footbridge is located near a busy pier where tourists embark on river trips.

It is also near the luxury Shangrila Hotel.

Police and bomb teams searching the area found scattered parts of the bomb.

Witnesses said dozens of people fled the area in terror when the bomb exploded.

"I was afraid there would be more bombs," food vendor Lamai Paraach, 51, told journalists.

Security camera footage showed a sudden blast of water dousing people on a walkway at the pier, as bystanders ran for safety.

Meanwhile, Thailand's prime minister Prayuth Chan-ocha warned in a televised address on Tuesday that it appears "there is still a group of individuals in our country who harbour and carry out their ill wishes against the nation."

"They aim to score political gain, or to destroy our economy, or tourism, or are driven by other motives," he said.

Mr Prayuth, a former army chief, asked Thais shocked over the carnage of Monday's huge bomb blast not to use social media in a way that could escalate conflict.

"Don't escalate conflicts with photos, audios, video or criticism or expressions of opinion," he said.

"Especially in the time that the investigation is still on-going as it may influence the case and mislead the public."

Mr Prayuth also moved to ease concerns of foreigners in the country following the blast in central Bangkok that left 22 dead and injured at least 120 more.

"As foreigners living in Thailand, including embassies, consulates, international organisations, the government would like to assure you that we will take care of your safety in lives, properties, and interests with the utmost effort and we will keep you informed of the news," he said.

Thai security forces believe CCTV footage may lead them to suspects of the first bombing that rocked the heart of the Thai capital on Monday evening.

"Today we have seen the closed-circuit footage, we saw some suspects, but it wasn't clear. We have to find them first." Prayuth Chan-ocha, the country's military chief told reporters.

Mr Prayuth said the suspects were believed to be from an "anti-government group based in Thailand's northeast" – the heartland of the kingdom's Red Shirt movement that opposes the military junta.

Thai media have published photographs of people who were in the area at the time who the police want to question, including a man wearing shorts and carrying a plastic bag.

Police said they believed he arrived at the shrine in a taxi and appealed to anybody who saw him to come forward. In one photograph he is carrying a backpack and in the other he isn't.

Broadcaster Thai PBS reported a man had left a "shoulder bag" inside a fence at the bomb site a minute before it went off.

PBS said a manhunt was underway "and all airports and border checkpoint were alerted to watch out for the suspect".