The Portuguese equivalent of the English Past Continuous is analogous to the Present Continuous Equivalent in Portuguese. The only difference is that the auxiliary verb Estar must be conjugated in the Past tense, not Present:

Present Continuous Equivalent

Estar [Present tense] + a + Main [Infinitive]
Neste momento estou a escrever um artigo.
Be [Present tense] Main [-ing form]
Right now, I am writing an article.

Past Continuous Equivalent

Estar [Past tense] + a + Main [Infinitive]
Ontem, [?] a escrever um artigo.
Be [Past tense] Main [-ing form]
Yesterday, I was writing an article.

As you can see, there’s a question mark replacing the Past tense of Estar. Why? As you may know, we have two kinds of Past tense in Portuguese: Preterite and Imperfect (Pretérito Perfeito and Pretérito Imperfeito). Whether we use one or the other, depends on the situation → Completed action vs. Ongoing action. Read on.

Completed action

As you may know, we use the Preterite to talk about completed actions: 

Estar [Preterite] + a + Main [Infinitive]
– O que fizeste ontem à noite?
– Estive a ver um filme.
– What did you do yesterday evening?
– I was watching a movie.

Notice that we can also use the main verb alone in the Preterite, just like in English we can use the main verb in the Past Simple:

Main [Preterite]
– O que fizeste ontem à noite?
–Vi um filme.
– What did you do yesterday evening?
– I watched a movie.

Is there any difference between these two variants? Nothing substantial, no. But there’s nonetheless a subtle nuance there. The 2-verb-version conveys more the notion of someone engaging in an activity for a while (more continuity and flow), whereas the latter is somehow more concise. 

Ongoing action

Now let’s try the same structure with Estar only now we conjugate it in the Imperfect instead. As you know, this tense conveys continuity:

Estar [Imperfect] + a + Main [Infinitive]
Estava a ver um filme quando me telefonaste.
I was watching a movie when you called me.

In this context, there’s a continuous action being interrupted by a punctual action: Someone was watching a movie when the telephone rang! Notice that we use the compound structure to convey the ongoing action (with Estar in the Imperfect) and the single verb (Telefonar) in the Preterite for the punctual action.

To summarize, the Past Continuous in English is used in distinct contexts concerning their temporal qualities: it can depict completed and ongoing actions in the past. Its Portuguese equivalent, however, will look different whether it is conveying one or the other type of action.