BANGKOK (Reuters) - A small “ping pong” bomb hidden in a plastic bag exploded in Bangkok on Friday, wounding a street cleaner, police said, after a spate of small bombs in the Thai capital raised speculation about opposition to military rule.

The blast came a day after authorities said they had arrested a suspect in connection with last month’s explosion at a military-run hospital in Bangkok that wounded more than 20 people.

“It was in a plastic bag hidden in a bush and city cleaning staff went to pick it out and opened it, that was when it exploded,” Kajornpong Jitpakpoom, the police commander in the city’s Thonglor district, told Reuters.

“It doesn’t appear to be a deliberate attack,” he added. “It could be related to students, who sometimes use small bombs like this and hide them.”

The devices are known as “ping pong” bombs as they are about the size of a table tennis ball.

Police bomb squad officers were at the scene, the team’s chief said, adding it was too early to provide details.

No group has claimed responsibility for several small blasts in Bangkok in recent months, but they have fueled speculation that opponents of military rule are making a show of defiance.

The military seized power from an elected government in May 2014, saying it had to step in to end months of at times bloody anti-government protests.

The military has promised to hold an election next year, but critics say it is intent on using a new constitution to maintain control over civilian governments.

‘HATES SOLDIERS’

Earlier on Friday, the military government suggested opposition to its rule was behind the hospital bomb attack last month, on the third anniversary of the coup.

“He hates soldiers,” Deputy Prime Minister Prawit Wongsuwan told reporters, referring to the arrested suspect, whom he blamed for bomb attacks in Bangkok three or four times since 2007.

The 62-year-old former electrical engineer was interrogated earlier on Friday at Bangkok’s 11th Army Circle base, used by the military as a temporary prison.

He has now been charged with premeditated murder, causing injury to others, and illegal possession of explosive materials and weapons.

Reuters was unable to contact the suspect and police said they were unable to comment on whether he had been granted a lawyer.

The military has the power to detain civilians at military camps without charge and rights activists say some face unfair trials in military courts, which the junta has designated for cases involving national security.

In August last year, explosions in several tourist towns on the same day killed four people and wounded dozens. No group claimed responsibility for the attacks, which some in the military government blamed on groups opposed to the junta.

Separatist insurgents in Muslim-majority provinces in the south also set off bombs, but rarely outside their region.

The hospital bombing followed a small blast that wounded two people on May 15 outside Bangkok’s National Theater and an explosion on May 5 in the city’s historic quarter.

The arrested suspect was involved in the two incidents as well as the hospital bombing, police chief Chakthip Chaijinda said late on Friday.

Junta chief Prayuth Chan-ocha has said more attacks and unrest could delay the general election, tentatively set for late 2018.

Thailand has been roiled for more than a decade by a struggle between populist political forces supported by poorer voters, and the old military-led establishment, largely backed by better-off voters based in Bangkok.