Police in Thailand have issued an arrest warrant for an “unidentified foreign man” seen leaving a bag minutes before a deadly bombing at a Bangkok shrine.

Police also released a sketch of the man they believe carried out Monday’s bombing and offered a 1 million baht (£18,000) reward for help leading to his arrest. But apart from the rough portrait, authorities had few solid leads.

At a news conference, the national police chief expressed uncertainty about the man’s origin. Somyot Poompanmoung said the suspect “looks like a foreigner” but “might have been in a disguise and wearing a fake nose” to conceal his identity.

Two days after the attack, which authorities have called the worst in Thai history, the open-air Erawan Shrine reopened to the public.

No one has claimed responsibility for the blast that left 20 people dead and more than 120 injured at one of the capital’s busiest intersections during evening rush hour.

A grainy security video shows the man, wearing a yellow T-shirt and shorts, sitting on a bench at the shrine, taking off a black backpack and leaving it behind as he stands up and then walks away. Time stamps on the video show he left the shrine 15 minutes before the explosion, which struck just before 7pm.

“If citizens or anyone can give us information or clues that lead to the arrest of this man, I have set a reward of 1 million baht,” Mr Somyot said, adding that police believe the bomber worked with accomplices.

“He didn’t do it alone, for sure. It’s a network,” Mr Somyot said. Police spokesman Lt Gen Prawut Thavornsiri said the security video appears to show two possible accomplices standing in front of the man who are also considered suspects.

Police composed the sketch based on the video and a description provided by a motorcycle taxi driver believed to have given him a ride on Monday night.

I have set a reward of 1 million baht. He didn’t do it alone, for sure. It’s a network

The sketch shows a young man in glasses with bushy, dark hair that is cropped at the sides. The warrant describes him as tall, with a pointed nose and thick lips.

He faces six charges, including conspiring to commit premeditated murder and conspiring to commit a bombing that resulted in death and severe injuries.

The attack has raised concerns about safety in a city that draws millions of tourists.

Prime Minister Prayuth Chan-ocha “is worried about the security of people and tourists in Thailand,” the police chief said.

Mr Prayuth has called the attack “the worst incident that has ever happened in Thailand” and vowed to track down those responsible.

The Erawan Shrine is a revered spot among Thais and tourists that transcends religion. It is dedicated to the Hindu god Brahma, but is extremely popular among Thailand’s Buddhists as well as Chinese tourists.

Tributes to London law student Vivian Chan killed in blast

A London law student was among at least 20 victims killed in a bomb blast at a popular shrine in Bangkok.

Vivian Chan, 19, is thought to have been travelling with a friend, who was also killed in the bombing which rocked the Thai capital.

The Foreign Office said it could not confirm whether Ms Chan was the British national from Hong Kong whose death had earlier been confirmed by Foreign Secretary Philip Hammond.

Professor Peter Crisp, Dean of Law School at BPP University in London, said: “Everyone at BPP University is devastated to hear of the loss of one of our students, Vivian Chan Wing Yan, in Bangkok. Our thoughts are with Vivian’s family and we are currently working to support them in any way we can.”

The Thai Society at the School of Oriental and African Studies in London said in a statement: “We... are deeply saddened to hear the news of Miss Chan ‘Vivian’ Wing Yan, a Law Student at SOAS, who passed away after a bomb blast in central Bangkok.

“We understand that Miss Chan, and her friend, were travelling in Thailand.

“According to sources, she recently posted a picture on an Instagram account of her food blog, @kittealuvfood, appreciating local delicacies.

“We would like to send our heartfelt condolences to her family and friends during this difficult moment.”

Writing on a Facebook tribute page, Ed Wawn posted: “Devastating news. Such a wonderful young women taken away so young. I really enjoyed teaching you Vivian... RIP.”

Sophia Hui said she was in “utter shock” to learn of her friend’s death, adding: “You were such a cheerful person and had one of the greatest smiles that I have seen.

“We, the Huis, are saddened with the loss. I still can’t believe this is real... RIP Vivian. You will always be remembered.”

Friends said Ms Chan was a fan of Hong Kong “Cantopop” music and one of her favourite singers, Joey Yung, posted a tribute on her Facebook page.

Below a picture of the pair together, the pop star wrote: “You have left a deep impression in me. Your wit, loveliness and your smile with dimples on the cheek.

“Thank you for being in my life.

“Our last conversation you said you would study hard and then we meet again. Never thought life could be that fickle... I hope you rest in peace.”

David Cameron said he was “deeply saddened” by the death of a British national in the bomb attack.

The Prime Minister posted on Twitter: “Deeply saddened to hear a British national was killed in the horrific Bangkok bombing.

“My thoughts are with her family and all those affected.”