The protesters, led by Chumpol Julasai, Nathapol Teepsuwan and Sakolthee Pattiyakul, took turns to verbally attack the government and demand Ms Yingluck's resignation.
They said the Feb 2 election was a flop because a large number of people did not turn out to vote. They also called for the military to stop complying with the government's orders.
About 1pm, they were joined by several hundred PDRC supporters led by Putthipong Punnakant and members of the State Enterprises Labour Relations Confederation.
Three layers of barbed wire fencing were erected to prevent the protesters from intruding into the defence offiices and about 20 soldiers were on security duty.
PM Yingluck reportedly had earlier left the office.
PDRC leader Suthep Thaugsuban said he would consult other PDRC core members about whether to lead protesters to blockade the houses of Ms Yingluck and some other cabinet members.
He was speaking after leading a large number of protesters relocating from the Lat Phrao intersection and Victory Monument stages to Lumpini Park, for "'safety reasons".
It was reported that the PDRC planned to expand the rally area from theNarathiwat Ratchanakarin intersection on Silom road to cover Ratchadamri road and join up with the Ratchaprasong rally site.