The first two blasts went off at around 7 a.m. near Building B of the Government Office Compound and Royal Thai Armed Forces Headquarters on Chaeng Wattana Road, Lieutenant Colonel Suban Athiset, Vice-Superintendent of Thung Song Hong Police Station, told Efe news.
He added that the police were "still investigating the type of bombs" and "there was no report of injury".
Another explosion went off an hour later down a street off Rama 9 Road, leaving two women injured.
Police told Efe that it was a home-made bomb and they believed it was planted by students from a technical institution nearby.
At around 8.30 a.m., there were two more blasts - one at the busy commuter skytrain station Chong Nonsi and one in a nearby parking lot - which injured two people.
The explosions occurred just before US Secretary of State Mike Pompeo was to deliver an address on the sidelines of the meeting.
The blasts came a day after a bomb threat at the police headquarters here.
The Thai government said in a statement that they were still investigating the explosions and urged the public not to panic.
"Regarding the bombs this morning, I would like to condemn those who are behind this, which destroy the peace and the image of the country. I have ordered the officials to take care of public safety and of those who have been affected by these incidents immediately," Thai Prime Minister Prayut Chan-ocha said on Twitter.
No group has so far claimed the attacks.
( With inputs from IANS )