\documentclass{article}
\usepackage{array}
\begin{document}
\begin{tabular}{| >{\centering}p{2.5cm}  | >{\centering}p{0.5cm}  | >{\centering}p{2.5cm} |>{\centering}p{2.5cm} |>{\centering}p{2.5cm} |>{\centering}p{2.5cm} |}
\hline
Abc & Bcd & A long cell with text that wraps around and is centered & long cell with text  & AAAlong cell with text that wraps around and is centered & SSSlong cell with text that wraps around and is centered \\
1 & 3 & qw & qe & sad &11\\
\hline
\end{tabular}
\end{document}

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If the centering is your only problem, then the question is a duplicate of Center column with specifying width in table (tabular enviroment)?. –  Martin Scharrer May 23 '11 at 16:36
@Strong: Welcome to tex.sx! +1 for your working minimal example. Would you mind to change your question title to something more descriptive (e.g. "Using \centering inside table column definition results in error message")? –  lockstep May 23 '11 at 16:40
@Martin: I'd rather think this question is a helpful "near duplicate", because it's about the error resulting from the changed definition of \\. –  lockstep May 23 '11 at 16:43
@Strong: I see that the same question appeared a second time. I assume this was a mistake, probably posting twice, so I've removed the duplicate. (For others, it was an exact copy of this one.) –  Joseph Wright May 23 '11 at 16:58
@Martin: I'm not sure this is exactly a dupe, as the question here is to do with why a problem arises from the 'obvious' approach. –  Joseph Wright May 23 '11 at 16:59

\centering changes the definition of \\ which breaks inside tabluars and arrays. You need to add \arraybackslash afterwards to correct this. You also should define a new column type if you need this more often:

\documentclass{article}
\usepackage{array}
\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}}

\begin{document}
\begin{tabular}{| C{2.5cm} | C{0.5cm} | C{2.5cm} |C{2.5cm} |C{2.5cm} |C{2.5cm} |}
\hline
Abc & Bcd & A long cell with text that wraps around and is centered & long cell with text  & AAAlong cell with text that wraps around and is centered & SSSlong cell with text that wraps around and is centered \\
1 & 3 & qw & qe & sad &11\\
\hline
\end{tabular}
\end{document}


See the related answer to Center column with specifying width in table (tabular enviroment)? for advanced declarations which support line-breaks and hyphenation of the first word.

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For narrow columns (such as the second one in the example code) it would be a good idea to add \hspace{0pt} to the column definition, to allow hyphenation of the first word. –  Gonzalo Medina May 23 '11 at 16:42
Dear Martin. When I ran it, one error appeared: úndefined control sequence'for command ''newcolumntype'' –  Strong May 23 '11 at 20:52
@Strong: It works for me. \newcolumntype is defined by the array package. –  Martin Scharrer May 23 '11 at 20:58