“Michel get out, austerity get out,” cried the four activists as the 38-year-old premier was beginning a speech in the southern city of Namur, according to several media present.
French fries, or chips in Britain, are a Belgian speciality, often served with a heavy dollop of mayonnaise.
The women, wearing masks and fully dressed unlike the usually topless Femen activists, were swiftly tackled by security personnel as Michel looked on, smiling in a mayo-stained suit.
After the incident, he restarted his speech apologising “that I smell of mayonnaise”.
The young premier, who took office in October, leads a coalition that intends to raise the retirement age in Belgium to 67, scrap plans for a usually automatic cost-of-living raise next year and introduce public sector cutbacks.
The plans sparked several rounds of strikes and demonstrations in the past weeks, including a march of more than 100,000 people in Brussels, which ended in violent protests that left dozens of police officers injured.
Food-related protests are a Belgian speciality.
In recent years, some of the world’s most powerful leaders, including former French president Nicolas Sarkozy and billionaire Microsoft founder Bill Gates, have received a custard pie in the face while on visits to the country.