ECR standing for European Court Ruling
Yes, I think these are ECJ references, as Bill says. Looking at where "Rec. p." and "Rec. pg." are used in Spanish docs, they do seem to refer to European cases.
I don't know whether there is a difference between "Rec. p." and "Rec. pg". You'd have thought so. But in any case, looking at the EUR-Lex site, which gives you official texts of European legislation and court rulings in the various Community languages, where Spanish versions use "Rec. p." or "Rec. pág." (much more often the former, in the EU versions; occasionally "Rec. pág.", but I've not yet found "Rec. pg."), the English version uses "ECR" for both.
For example:
"(sentencias de 4 de julio de 2006, Adeneler y otros, C‑212/04, Rec. p. I‑6057, apartados 54 y 57; de 7 de septiembre de 2006, Marrosu y Sardino, C‑53/04, Rec. p. I‑7213, apartados 40 a 43, y Vassallo, C‑180/04, Rec. p. I‑7251, apartados 32 a 35, y Del Cerro Alonso, antes citada, apartado 25)."
"(Case C-212/04 Adeneler and Others [2006] ECR I-6057, paragraphs 54 to 57; Case C-53/04 Marrosu and Sardino [2006] ECR I-7213, paragraphs 40 to 43; Case C-180/04 Vassallo [2006] ECR I‑7251, paragraphs 32 to 35, and Del Cerro Alonso , paragraph 25)."
http://eur-lex.europa.eu/Notice.do?mode=dbl&lang=es&ihmlang=...

"[5] Sentencia de 10 de mayo de 1995, Alpine Investments, asunto C-384/93, Rec. pág. I-1141."
"[5] Case C-384/93 Alpine Investments [1995] ECR I-1141."
http://eur-lex.europa.eu/Notice.do?mode=dbl&lang=en&ihmlang=...


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Note added at 17 hrs (2011-01-07 04:00:57 GMT)
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In response to the points raised by Bill in the discussion, I think it is clear that normal, though perhaps not universal, practice in English is to refer to European case law with the reference "ECR", the standard abbreviation for the European Court Reports (called "Rec." in French and Spanish, standing for "Recueil" and Recopilación" respectively). To quote the Exeter website where the reports themselves can be consulted (cited by Bill above):
"A full citation must always include the following elements:
1. The case number (C-91/92) shows when the case was added to the Court’s register (1992 in this example)
2. The names of the parties involved (Faccini Dori v Recreb)
3. The year in which the case was judged (1994 in this example)
4. ECR = European Court Reports (the official series of reports)
5. Section I (from 1989) = Court of Justice; (Section II = Court of First Instance)
6. The page number at which the report starts. The page reference is the same for all language versions of the reports, except for those countries which have joined since 2004."
The page number is given just as a without "p.", as is increasingly the norm in English in all sorts of bibliographical references, incidentally.
It is striking that text books in English on European law use this form of reference.

The other point which has become clear, I think, is that the use of "p." or "pg." (or indeed "pág.") in Spanish citations, despite initial impressions, is in fact arbitrary: all these abbreviations mean "page".