Sectarian violence against minority Rohingya Muslims in the predominantly Buddhist nation has killed hundreds in the past year, and uprooted about 140,000.
Observers fear the violence presents a threat to Myanmar's transition to democracy because it could encourage security forces to re-assert control.
A small group of protesters demonstrated outside Parliament as Cameron met President Thein Sein at 10 Downing Street.
Thein Sein, a former general who retains close ties to the military, has introduced democratic changes after decades of authoritarian rule that had led to international isolation. Cameron said Britain welcomed the reforms and looked forward to free and fair elections in 2015.
Ricken Patel, executive director at human rights campaign group Avaaz, said the attacks on the Rohingya "should be ringing alarm bells" everywhere.
"Cameron has a responsibility to use all his diplomatic leverage to get real, concrete measures from President Thein Sein to protect these groups," Patel said.