The allegations that “Russian mercenaries” are present in Libya are nothing more than rumours.
The EU has not presented any specific evidence and has not even asked Russia for any proof on its allegations. Furthermore, the “Russian threat” is a unifying factor for Western countries.
There is a constant effort by Moscow to deny the fact that pro-Kremlin outlets are engaged in a concerted disinformation campaign aiming to expand the impact of coronavirus in the EU and its member states. This has been exposed in various media reports (see here and here). There has also been extensive analysis of the stories and the narratives behind this effort. Some of the disinformation stories focus on promoting the narrative that coronavirus was created in a laboratory while others aim to discredit the efforts made by EU member states to contain COVID-19. Another narrative is concentrated in highlighting the cynical and selfish way some member states treated their EU partners, undermining solidarity within the Union. This ignores the significant efforts made by EU institutions in finding common solutions in the fight against coronavirus (see here and here). In April, Facebook removed 46 Pages, 91 Facebook accounts, 2 Groups, and 1 Instagram account for violating its policy against foreign interference which is coordinated inauthentic behavior on behalf of a foreign entity. The activity originated in Russia, the Donbass region in Ukraine and the Crimean Peninsula. The network posted about geopolitical and local news including topics such as the military conflict in Ukraine, the Syrian civil war, the annexation of Crimea, NATO, US elections, and more recently the coronavirus pandemic. The activity was linked to individuals in Russia and Donbass, and two media organizations in Crimea — NewsFront and SouthFront. See EUvsDisinfo analysis here.