The chief military advisor of Thailand's anti-government protestors sustained head injuries after an explosion and bursts of automatic gunfire in Bangkok today.

Khattiya Sawasdipol was admitted to an intensive care ward after being shot, the state Narenthorn Emergency Medical Service reported.

It had no other details.

Khattiya, better known as ‘Seh Daeng’ (Commander Red) enjoys a cult following among some red shirts and soldiers, but has been dubbed a ‘terrorist’ by Thailand's government, which accuses him of involvement in dozens of grenade attacks that have injured more than 100 people.

The army had earlier said it was planning a huge lockdown around the fortified encampment of the red shirts, who have defied warnings to end their five-week occupation of an upmarket Bangkok shopping district.

Armoured vehicles were to be deployed and roads shut surrounding thousands of defiant protesters, forcing businesses to evacuate workers.

The army said its armoured vehicles will bolster checkpoints, stopping protesters from entering the area, and urged businesses on roads leading into the protesters' 3sq-km fortified encampment to close.

Army spokesman Sansern Kaewkamnerd said authorities tasked with resolving the crisis will also seek cabinet approval to invoke a state of emergency in 15 northern and north-eastern provinces.

The mostly rural and urban poor protesters refused to leave as their leaders challenged the government from behind medieval-like walls made from tyres and wooden staves soaked in kerosene and topped by razor wire.

The government estimated the crowd size at 10,000 but Reuters put it at more than 20,000.

‘We will send out groups to surround these vehicles to prevent them from advancing,’ Jatuporn Prompan, a protest leader, told supporters.

‘We believe the army will try to crack down this evening or tomorrow morning.’

The protesters have said they would only disperse if a deputy prime minister faces criminal charges over a deadly April clash between troops and protestors.