BANGKOK (Reuters) - Two pipe bombs exploded outside a luxury shopping mall in Bangkok on Sunday in an attack which Thai police said was aimed at raising tension in a city living under martial law.

The bombs caused little damage but were the first to shake the Thai capital since the military seized power in May to end months of sometimes deadly street protests.

"There were improvised explosive devices detonated by a digital clock," said Police Lieutenant General Prawut Thawonrsiri, a spokesman for the Royal Thai Police.

The motive behind the attack appeared to be to create panic rather than take lives, Prawut said. The police have yet to identify the bombers, he added.

Two improvised explosive devices were placed behind power transformers on an elevated walkway linking the overhead rail line to the upmarket Siam Paragon mall in central Bangkok, police said.

Political tension in Thailand has been high since a national assembly hand-picked by the junta last month banned former Prime Minister Yingluck Shinawatra from politics for five years.

The same day, the country's Attorney General said she would face charges and a possible jail sentence for negligence.

The decisions angered supporters of Yingluck and her exiled brother Thaksin, although there has been little sign of a return to the street protests that have dogged Thailand for the past ten years. Military repression has snuffed out dissent since the coup.

The junta last week summoned senior politicians critical of the government's handling of Yingluck's cases for a dressing down.

Ten years of turbulent politics in Thailand have pitted Yingluck and Thaksin, himself a former prime minister, and their rural support base against the Bangkok royalist-military establishment that sees the Shinawatras as a threat and reviles their populist policies.

This was the first bomb in the Thai capital since before the coup, Army Lieutenant General Kampanart Ruddit told reporters.

Sporadic violence during the six months of street protests that preceded the May 22 coup claimed almost 30 lives, with scores more injured.

The explosions on Sunday showed that martial law could not yet be lifted, said Kampanart, who is responsible for the Bangkok area.