MegaMouth is to mingle with shoppers and office workers all week, asking for their views about world poverty, climate change and the recession.
Volunteers must provide comments that are no more than 140 characters in length, which is the limit for a Twitter message.
With his megaphone, MegaMouth will yell-out the best slogans collected.
MegaMouth's support team includes a camera crew who will feed video clips and photos to ActionAid's website, and a 'twitterer' who will give a running text-message commentary of the action on MegaMouth's Twitter page.
On Saturday March 28, MegaMouth will join supporters of ActionAid for the Put People First march from Victoria Embankment to Hyde Park.
The march is intended to give a clear message about jobs, justice and climate change to Gordon Brown, Barack Obama and other leaders attending the G20 summit on April 2.
Emma Harbour, ActionAid campaigner, said: "Banks are crashing, world hunger is growing, and climate change is going out of control. It's a huge crisis, but it is also a once-in-a-lifetime opportunity to change the system so it puts people first.
"We want everyone to join ActionAid and the 125 other organisations on Saturday's march. People are angry about the situation. We are giving them a chance to be creatively angry, by coming up with a snappy slogan that will catch the attention of the busy leaders at the G20 summit."
The idea and the technology behind MegaMouth were developed for ActionAid by the digital marketing agency Nonsense.