Many if not most of you will have come across a  photograph like this of the Venerable Servant of God Pius XII  celebrating Low Mass at the papal altar of St. Peter's Basilica. 

Since the solemn papal Mass was an incredibly complex ritual involving a very large number of people holding offices at the Curia and the papal court, including highranking prelates, it was only celebrated on rare occasions (for a brief description, see here). An alternative - although an extraordinary one - was the public celebration of a Papal Low Mass. At a Papal Low Mass (which was usually celebrated at a portable altar set up in one of the rooms of the Apostolic Palace and is distinct from the private Mass the Pope said in his private chapel), the Pope is assisted by two bishops and four papal Masters of Ceremonies. Chamberlains (cubicularii) served as torchbearers. The pontifical canon was used, as was the bugia.
Here is a video of such a Mass. The Mass was celebrated on 14 May 1942, the Feast of the Ascension, and the occasion was the Silver Jubilee of the Pontiff's episcopal consecration (you can find the text of the papal sermon in AAS 34 (1942), 167).

Since the solemn papal Mass was an incredibly complex ritual involving a very large number of people holding offices at the Curia and the papal court, including highranking prelates, it was only celebrated on rare occasions (for a brief description, see here). An alternative - although an extraordinary one - was the public celebration of a Papal Low Mass. At a Papal Low Mass (which was usually celebrated at a portable altar set up in one of the rooms of the Apostolic Palace and is distinct from the private Mass the Pope said in his private chapel), the Pope is assisted by two bishops and four papal Masters of Ceremonies. Chamberlains (cubicularii) served as torchbearers. The pontifical canon was used, as was the bugia.
Here is a video of such a Mass. The Mass was celebrated on 14 May 1942, the Feast of the Ascension, and the occasion was the Silver Jubilee of the Pontiff's episcopal consecration (you can find the text of the papal sermon in AAS 34 (1942), 167).

 inundado por um mistério de luz que é Deus   e N´Ele vi e ouvi -A ponta da lança como chama que se desprende, toca o eixo da terra, – Ela estremece: montanhas, cidades, vilas e aldeias com os seus moradores são sepultados. - O mar, os rios e as nuvens saem dos seus limites, transbordam, inundam e arrastam consigo num redemoinho, moradias e gente em número que não se pode contar , é a purificação do mundo pelo pecado em que se mergulha. - O ódio, a ambição provocam a guerra destruidora!  - Depois senti no palpitar acelerado do coração e no meu espírito o eco duma voz suave que dizia: – No tempo, uma só Fé, um só Batismo, uma só Igreja, Santa, Católica, Apostólica: - Na eternidade, o Céu!
inundado por um mistério de luz que é Deus   e N´Ele vi e ouvi -A ponta da lança como chama que se desprende, toca o eixo da terra, – Ela estremece: montanhas, cidades, vilas e aldeias com os seus moradores são sepultados. - O mar, os rios e as nuvens saem dos seus limites, transbordam, inundam e arrastam consigo num redemoinho, moradias e gente em número que não se pode contar , é a purificação do mundo pelo pecado em que se mergulha. - O ódio, a ambição provocam a guerra destruidora!  - Depois senti no palpitar acelerado do coração e no meu espírito o eco duma voz suave que dizia: – No tempo, uma só Fé, um só Batismo, uma só Igreja, Santa, Católica, Apostólica: - Na eternidade, o Céu!