The move follows confirmation earlier this week that Vincent Bollore has now amassed 22% of the company.
Bollore had been buying shares in Havas during the summer, but his intentions for the company remain a mystery.
Frere began building his stake in Havas last month when his holding in the Euro RSCG-owning network moved to 2% and then up to 4%.
There has been response from Bollore, who is the biggest shareholder in Havas with his 22% stake.
Analysts believe that the arrival of Frere increases the pressure on the Havas management team, led by chief executive Alain de Pouzilhac.
In a research note, Exane BNP Paribas wrote: "Mr Frere's arrival in Havas' capital is likely to increase the pressure on Havas' management and sustain speculation on the company's disposal or break-up."
Havas today confirmed Frere's 5% stake and described it as "good news". Its response indicates that it could see the Belgian as a supporter of the management team against Bollore, who has demanded two seats on the board and did not support De Pouzilhac in his efforts to buy Grey Global Group.
Bollore, who controls 23.14% of the Havas voting rights, is expected to make clear his intentions towards Havas later today, or by Monday at the latest, according to reports.
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