How seriously should we take opera seria, that 18th-century celebration of excess in excelsis? The party line is that you take it very seriously indeed. Respect the idiom, don't guy it with feathers and flummery, make those sumptuously decorated arias and tortuous plots work for you. Well, that's the theory. At London's Shaftesbury Theatre at the moment, there is an unprecedented opportunity to see not one, but two specimens of the seria idiom, both in wilfully intemperate stagings: Lindsay Posner's Royal Opera production of Handel's Giulio Cesare; and Mozart's Lucio Silla in Brigitte Fassbaender's Opera For Europe production.

Opera seria characters don't have inner lives, they have eruptions of emotion; they don't have relationships, they have confrontations. Posner's production, in Joanna Parker's designs, duly offers poster-paint characterisation that frequently enters the comedy zone of Carry On Cleo: Brian Asawa's Tolomeo is carried on and off stage, clutching, not a comfort blanket, but a comfort crocodile. Moment to moment, the effect seems ludicrous, but by the end of a long evening (lengthy cuts have been made since the production was new) the logic has worked, and we feel for these one-dimensional entities, nowhere more so than when Ann Murray's Caesar, cavorting like a demented samurai, swears vengeance and mayhem on all.

Mozart's Lucio Silla, written when the composer was 16, is less substantial, though not much shorter than Giulio Cesare (again cuts have been made). It wasn't opera's job to question the political status quo, but to celebrate it, so that here the Roman tyrant Sulla has suddenly, ex nihilo, to turn benign. It's brave of Opera For Europe to tackle this unwieldy curio. The touring arm of the European Opera Centre (based in Manchester) aims to present young European singers in touring productions, of which Fassbaender's is the first.

In Bettina Munzer's designs, Sulla becomes a small-town tough guy, his senate a bunch of doddering war veterans. Fassbaender, once no mean Mozartian as a singer herself, here decrees that the da capo arias need not only vocal, but dramatic ornamentation. Cinna's opening aria finds Dorothee Tsalos shining Cecilio's boots; Anke Vondung's Cecilio proceeds to shave while singing. This can get tiresome but the production pursues its logic relentlessly, and there are laughs where least expected.

Vocally, the women come off best. Vondung successfully negotiated the precipitate leaps demanded of Cecilio, and Tsalos made a cheeky Cinna, forever thwarted in attempts to assassinate Sulla. As the tyrant's sister Celia, Orla Boylan showed considerable comic gifts, but the most impressive performance came from Eva Oltivanyi's Giunia, looking like Frida Kahlo and singing cleanly and clearly. Ruben Martinez strutted manfully as Sulla and, despite a slightly veiled tone, his tenor coped with most of what Mozart threw at him. The Shaftesbury's boxy acoustic does no favours for young voices, and under Fabrizio Ventura the orchestra sometimes sounded undernourished, apart from a hyperactive harpsichord, yet there were gorgeous moments. Mozart's music is formulaically lovely, and often more besides: the brass writing, in particular, introduces orchestral shadows that prefigure Don Giovanni.

Opera seria is sturdy enough to take a battering now and then; and what is perhaps surprising (though the directors would say "I told you so") is that, once you step into their imaginative world, there is a consistency in these stagings that, while not telling the whole story, at least maintains the narrative momentum. Not all be-toga'd productions can say that.

`Lucio Silla' 7.30pm tonight, `Giulio Cesare' 6.30pm tomorrow, Shaftesbury Theatre, London WC2 (0171-304 4000). `Lucio Silla' is also touring to Basingstoke, Birmingham, Oxford, Llandudno, Edinburgh, Norwich, Dublin, Cork and Belfast