protest
Activists often protest protest rallies organized by opposing groups.
The sentence describes activists frequently demonstrating against rallies organized by their opponents.

Meanings
to express strong objection or disagreement, especially publicly
- Citizens protested the new tax.
- She protested loudly when the verdict was read.
a public expression of objection or disapproval, often a demonstration or rally
- Thousands joined the protest in the city square.
- The protest lasted three days.
Word origin
From Latin prōtestārī ('to declare publicly'), formed from prō- ('publicly, before') + testārī ('to bear witness'), from testis ('witness'). The same root produces 'testify', 'testament', 'testimony', 'attest', and 'contest'. The verb-noun stress alternation follows the trochaic noun rule.
Fun fact
The Latin prōtestārī originally meant 'to declare publicly' — neutral in tone. Only later did 'protest' acquire its specifically negative meaning of 'objection.' You can still see the neutral sense in 'protestation' and 'Protestant' (one who 'protests' or publicly declares a position) — historically religious dissenters who declared their differences with Rome.