Collective phenomena with universal properties have been observed in many complex systems with a large number of components. Here we present a microscopic model of the emergence of scaling behavior in such systems, where the interaction dynamics between individual components is mediated by a global variable making the mean-field description exact. Using the example of financial markets, we show that asset price can be such a global variable with the critical role of coordinating the actions of agents who are otherwise independent. The resulting model accurately reproduces empirical properties such as the universal scaling of the price fluctuation and volume distributions, long-range correlations in volatility and multiscaling.PACS numbers: 89.75. Da,89.65.Gh,05.65.+b,05.40.Fb Universal scaling behaviour is an emergent property of many complex systems [1]. In such systems, the interactions between a large number of individual components yields macro-scale collective behavior with features that are almost invariant across different spatial and temporal scales [2]. A financial market provides a general and useful paradigm of such a system, since it involves a large number of agents whose actions are subject to internal and external influences, such as information about the state of the market as provided by market indices [3]. Despite this complexity, the availability of a large volume of high-quality data for analysis has enabled the identification of well characterized statistical properties [4,5]. These properties, including the fat-tailed distribution of relative price changes [6,7] and intermittent bursts of large fluctuations that characterize volatility clustering [8], appear to be universal: they are invariant across different markets, types of assets traded and periods of observation [9]. More generally, the question of how universal features emerge from collective behavior in systems with many components is not restricted to the purely economic domain. Thus, new approaches to understanding the behavior of financial markets may contribute to the understanding of the physics of nonequilibrium steady states in general.Mainstream economic theories for price fluctuations of financial assets typically assume the efficient market hypothesis [10]. According to this, price variations reflect changes in the fundamental (or "true") value of the assets. However, detailed analysis of data from actual markets show that much of the observed price variation cannot be explained solely in terms of changes in economic fundamentals [11]. The absence of a strong correlation between large market fluctuations and purely economic factors leaves unresolved the question of why markets are so volatile. As the dynamics of markets are a result of the collective behavior of many interacting constituents, models based on statistical physics have been proposed to explain the observed universal behavior [12][13][14][15]. Most such models consider explicit interactions between agents to reproduce a very limited set of the universal emp...